In the hills and hollows of Mingo County, W.Va., where unemployment is nearly triple the national average, it’s coal. On the southwest side of Chicago, where the landscape couldn’t be more different but the economic fears are much the same, it’s Oreo cookies. Elsewhere, it is cars, computers and air-conditioners.

And whether it is through rolling back regulations, imposing tariffs or making some none-too-discreet phone calls from the Oval Office to the C-suites, has vowed to bring back the vanishing jobs of miners, bakers and assembly-line workers, beginning on Day 1 of his administration.

“It’s going to happen fast,” Mr. Trump, the presumptive Republican nominee for president, recently told a cheering crowd in Charleston, W.Va. “This is so easy.”

[Video: FULL Event: Donald Trump’s HUGE RALLY in Charleston, WV (5-5-16) ]

FULL Event: Donald Trump’s HUGE RALLY in Charleston, WV (5-5-16)
Video by Right Side Broadcasting

If only that were true.

For all of the appeal his message might have for residents there (Mr. Trump captured almost 90 percent of the vote in Mingo County), much of what he is promising to do — on his own, and through congressional legislation — couldn’t be accomplished in the first 1,000 days of a Trump administration, much less the first 100.

For example, Mr. Trump has suggested easing clean-air regulations enacted by Democrats and Republicans alike that have hurt the coal industry. But King Coal is unlikely to ever recapture market share lost in recent years to made cheap by the fracking boom, not to mention fast-growing alternative energy sources like wind and solar.

Nor could Mr. Trump, a billionaire businessman, force steel makers to buy coal from Appalachia to heat furnaces in Asia, Europe and North America that have been idled by weak demand.

“I will not say he can’t do anything, but it’s very unlikely he’ll be able to restore coal to where it was,” said John Deskins, director of the West Virginia University Bureau of Business and Economic Research. With production and employment in the mines down by about a third since 2008, Mr. Deskins said, “even in our most optimistic scenario, we don’t expect a big bounce back.”

Of course, a Trump presidency is far from certain. And any president’s sway over the national economy is debatable.

But a big part of what has gotten Mr. Trump this far are his outsize promises. And while the case of coal and clean air illustrates the limits of a president’s power in the economic arena, there are other places where Mr. Trump would have considerably more room to maneuver.

Like much of his speechifying, Mr. Trump’s economic and business agenda is a mixture of opening bids and dog-whistle messages as well as some nuts-and-bolts proposals he might be able to put in effect as president, even without congressional approval. Sorting out what’s what, though, isn’t simple or safe. Just ask the pundits who wrote off Mr. Trump’s candidacy as a sideshow even as he won one primary after another.

For his part, Mr. Trump warns it would be a mistake to underestimate his ability to change the rules of the game.

“In my whole life, I’ve gotten things done,” he said in an interview on Saturday. “Whether it’s getting a city built on the West Side of Manhattan or getting zoning board approvals, my whole life has been finding a consensus.”

But when it comes to companies moving jobs out of the United States, he said a tougher tone was in order.

“I’m not Obama, and there are stupid people in our government,” he said. “With me, there will be consequences if you move, and the consequences will be severe.”

In terms of coal and recovering all of those vanished jobs in West Virginia, Mr. Trump acknowledged that price competition from cheaper natural gas was fierce.

“But coal is still less expensive, and it has a major place in terms of energy,” he said. And regulatory relief — whether in terms of environmental rules or workplace safety — would benefit the industry, he said.

“I have become very well versed on coal,” Mr. Trump said. “The regulations are brutal, and they are sending inspectors into the mines two and three times a day. Even the miners say it’s out of control.”

In law firms, corporate boardrooms, lobbyist watering holes and think tanks in Washington and beyond, experts are quietly assessing what a Trump presidency might actually look like in practice. Or to put it more bluntly, business is business. So whether they find Mr. Trump politically abhorrent or a welcome antidote to the status quo, these insiders also want to know what he might mean for their bottom line.

National debate: “Trump would put the bully in bully pulpit.”

Perhaps the greatest opportunity for Mr. Trump lies in the extraordinary ability of any president to direct the tone and contours of the national debate, especially on issues he has highlighted, like how American companies should treat their American workers. And unlike most past Republican candidates, Mr. Trump has not been reluctant to criticize big business on that topic.

“Donald Trump would put the bully in bully pulpit,” said Seth Harris, who served as a top official in the Labor Department under Democratic presidents.

Immigration: “Where the president’s power is least restricted.”

Consider immigration, . He might well be able to deliver on one of his central campaign promises: handing out fewer green cards to foreign workers, or denying a place to H-1B visa holders unless their American employers agree to pay them more.

“There’s a reason why so many of Trump’s proposals revolve around the idea of denying visas to people,” said William A. Stock, a lawyer in Philadelphia who is the president-elect of the American Immigration Lawyers Association. “That’s where the president’s power is least restricted, if he asserts it.”

The Immigration and Nationality Act of 1952, Mr. Stock said, gives the president “the ability to suspend the entry of foreign nationals whose entry is deemed to be detrimental to the interests of the U.S.”

Noting that recent administrations have invoked the law to forbid entry to individuals from the former Yugoslavia accused of human rights violations, as well as to Cuban and Iranian government officials, Mr. Stock said, “This is a tool that is pretty powerful, but it’s usually been used in more limited instances.”

Financial reform: “When it comes to regulation, people are policy.”

And what about “” the 2010 Dodd-Frank financial law, as Mr. Trump promised this month?

Repealing the law might be difficult, but experts who helped draft the regulations agreed that Mr. Trump could defang them through selective enforcement — appointing different officials to the Federal Reserve and the Securities and Exchange Commission — and by issuing new executive orders.

A President Trump would have wide latitude to name his own people to oversee less conspicuous but powerful agencies like the Commodity Futures Trading Commission, the Consumer Financial Protection Bureau and the Office of the Comptroller of the Currency, and to seed Treasury and other government departments with like-minded officials.

“When it comes to regulation, people are policy,” said Aaron Klein, a top official at the Treasury Department from 2009 to 2012 and helped draft the Dodd-Frank bill. “Regulators who don’t want to regulate can thwart the will of Congress.”

But the president’s reach isn’t unlimited. Nor is it speedy. And however stupendous Mr. Trump’s deal-making skills may be, the forces of gridlock in the nation’s capital are no less awesome.

“The system doesn’t change,” said Tom Korologos, a longtime Republican strategist and an adviser at the law firm DLA Piper in Washington. “Trump can say, ‘I’m going to repeal this or I’m going to repeal that,’ but it’s going to take longer than 100 days.”

Mr. Korologos, who served on the transition teams of President Ronald Reagan and President George W. Bush, isn’t supporting Mr. Trump this year. He added that while the presumptive Republican nominee’s promises might make “for nice talking points, it will be harder than he thinks.”

Tax cuts: A Republican Congress is unlikely to accept a $10 trillion deficit over 10 years.

That’s especially true when it comes to the macroeconomic picture. Mr. Trump claims his proposal for sweeping tax cuts for individuals and businesses wouldn’t bust the budget and could lift the economy’s annual growth rate to 6 percent.

More sober-minded experts say that’s magical thinking, a 2016 version of “voodoo economics,” as President George H. W. Bush famously described what came to be known as Reaganomics in the 1980s. They estimate Mr. Trump’s tax plan could create a $10 trillion shortfall at the Treasury over the next decade, and note that annual economic growth has topped 5 percent only once in the last 35 years.

“I have a hard time believing that even a Republican Congress would enact his tax plan, because it would create enormous deficits,” said William G. Gale, an economist under the first President Bush who is now co-director of the Tax Policy Center in Washington.

Mr. Trump responded that “those tax cuts go hand in hand with many other things.”

As a result of getting tough on trade policy and what he considers currency manipulation by China, Mr. Trump argues that more jobs would return to the United States, which in turn would spur economic growth and therefore tax revenues.

Whether or not that’s realistic, any significant alteration in trade or tax policy means winning over the House Ways and Means Committee and the Senate Finance Committee, both of which zealously guard their turf. “This is what makes them tick,” Mr. Korologos said. “It’s going to take more than just a bunch of rhetoric to change things.”

Labor: An ebb and flow when administrations change hands in Washington.

Like Mr. Korologos, Mr. Harris, the former Labor Department official, has seen the ebb and flow when administrations change hands in Washington. Except he was on the opposite side of the aisle, having served on the Democratic transition teams after President Bill Clinton was elected in 1992 and after President Obama’s victory in 2008.

At Labor, where he was a top adviser under Mr. Clinton, and then as deputy secretary in the Obama administration, Mr. Harris saw firsthand just how much influence the president could have.

Tasked with enforcing regulations on whether companies are abiding by minimum wage requirements, restrictions, affirmative action guidelines, workplace safety rules and a web of other laws, Mr. Harris said, the Labor Department has never been very popular with big business or the Republican candidates it supports.

And when George W. Bush succeeded Bill Clinton, the change was swift, Mr. Harris said. Budgets were cut, and travel to inspect factories and mines decreased. There was less emphasis on enforcement, going into workplaces and punishing violations, he said, and more on educating employers about potential infractions.

After Mr. Harris returned to the Labor Department in 2009, budgets were increased, and surprise workplace inspections were resumed.

“We investigated more deeply, enforced procedures, and were more likely to find violations,” he said. “There was no executive order or action by Congress or the White House. We just did it. The laws existed — it was a matter of whether you were going to solve the problem aggressively or be more passive.”

Republicans, including Mr. Trump, have long argued that regulatory zeal kills jobs and undermines economic growth. Democrats maintain it is necessary to protect employees from abuses and risks, while imposing checks on the power of big companies.

Both arguments contain elements of truth. But Mr. Harris said that what was critical was that rank-and-file civil servants quickly adapted to the message coming down from the Oval Office.

“The president sets the tone for how our government is going to relate to our country,” said Mr. Harris, who is now a lawyer in private practice in Washington. “How the laws are implemented and enforced, how the money is spent and how motivated people are to do their job.”

Health care: “Selling insurance isn’t like selling credit cards.”

Dodd-Frank isn’t the only big Obama-era law Mr. Trump has talked about repealing. The other prominent target of Mr. Trump’s ire — the Affordable Care Act — would be trickier to dispose of.

Even if Republicans controlled the White House and both houses of Congress, simply repealing the Affordable Care Act, known as Obamacare, could bring on a political backlash from the who are directly covered under the law, not to mention millions more who benefit from provisions like those that allow young adults to be covered under their parents’ policy and those that prevent insurers from dropping coverage for pre-existing conditions.

Moreover, Mr. Trump displays a lack of understanding of how health insurance works. For example, one specific change in the A.C.A. that Mr. Trump, along with other Republican candidates, has called for — allowing insurers to sell — is permitted under the law now.

The problem, said Nicholas Bagley, a professor of law at the University of Michigan, is that insurers don’t want to do it because it’s not practical. “Selling insurance isn’t like selling credit cards,” Mr. Bagley said. “Out-of-state health insurers don’t have networks of doctors and hospitals in place. The problem isn’t legal restraints, and it’s not a solution to what ails the health care economy.”

Mr. Trump responded that deregulation is still the answer, despite the skepticism of experts like Mr. Bagley.

“The problem is rules, regulations and restrictions,” he said. “You’re going to have rate increases that are catastrophic, and the deductibles are so high people can’t even use the health care. If it was really open and you got competition going, amazing things will happen.”

Environment: Sending “a chill down the spine” of people who enforce the laws.

Environmental regulation offers a more promising target for Mr. Trump, who has called the Environmental Protection Agency a laughingstock and a disgrace, and has promised to cut its budget.

In fact, in the absence of congressional action, the Trump administration could target many other environmental regulations by simply going after the people who enforce them, said Don Barry, who spent two decades at the Interior Department and directed the United States Fish and Wildlife Service under President Clinton.

Decades later, Mr. Barry still recalls how a round of firings in the Interior Department at the dawn of the Reagan administration signaled a turnabout in policy. “The single biggest thing they can do is send a chill down the spine of career survivors in the bureaucracy and bring things to a halt,” he said.

Similarly, a Trump administration might be willing to approve state environmental plans that are more friendly to fossil fuels, said Kevin Book, head of research at ClearView Energy Partners. Proposed, but not completed, clean-water regulations aimed at restricting mountaintop-removal mining for coal in West Virginia could be eased, he said.

Trade: Trump can punish countries, but jobs are unlikely to return.

Still, even if Mr. Trump were to win in November and roll back some environmental regulations, it wouldn’t significantly alter the fate of Mingo County and much of the nation’s depressed coal industry.

“Taking a more lenient stance on clean-air or clean-water rules would do nothing to help coal demand,” Mr. Book said. “Coal-fired power plants have been mothballed and are being disassembled, and that capacity is gone. It’s not even close to something you could reverse by administrative fiat.”

If Washington’s ability to gridlock even the most ambitious of presidential agendas were to block Mr. Trump’s White House plans, he would still have the bully pulpit, of course. And unlike Theodore Roosevelt, who coined that term, Mr. Trump probably wouldn’t speak softly.

Still, that might not be enough to keep Oreos baking in Chicago. Nabisco’s bakery there isn’t shuttering completely as Mr. Trump has suggested, but some Oreo production will begin shifting to Salinas, Mexico — where Mondelez, Nabisco’s corporate parent, has invested to build new lines. Oreos will also continue to be produced at Nabisco bakeries in Virginia, New Jersey and Oregon.

Announced in July 2015, the Nabisco layoffs in Chicago continued during the Illinois primary in March, despite jabs from Mr. Trump and both Democratic presidential contenders, Hillary Clinton and Senator Bernie Sanders.

United Technologies is similarly unlikely to rethink Carrier’s move out of Indianapolis. And in an era when Apple is willing to fight the Obama administration’s efforts to unlock an iPhone on national security grounds, more corporate defiance may become the norm.

Blanketing countries from China to Mexico with import duties would be a tall order without congressional approval. The White House, though, does retain plenty of leeway to punish countries it thinks are engaged in currency manipulation, dumping of products below the cost of production and other free-trade abuses.

The problem for Mr. Trump — or any president who wants to get tough on trade violators — is that, in the global economy, imposing tariffs on competitors abroad could have serious economic consequences at home by sharply raising prices on imported goods.

Cheaper flat-screen televisions, computers, clothes, furniture and other products from Walmart, Amazon and elsewhere have been a rare bright spot for struggling working- and middle-class Americans. And trade wars cut both ways: Retaliatory tariffs on American-made products from countries like China would prompt howls of pain at still strong domestic manufacturers like Caterpillar and Boeing.

But for all the uncertainty around what might happen after Jan. 20, 2017, the lawyers and lobbyists who are now trying to gauge a Trump presidency are already emerging as winners. Mr. Trump’s much-talked-about unpredictability, Mr. Korologos said, is proving to be a boon for Washington’s legions of lobbyists.

“Corporate America hates the unknown,” he said. “God only knows what the hell Trump is for. And corporate America is going to want someone to save them.”