WASHINGTON — The stunning election of Donald J. Trump as president and the re-election of a Republican-controlled House and Senate will usher in sweeping change to the nation’s policy course that will most likely amount to a repudiation of much of President Obama’s legacy and will reorient the United States’ position on the world stage.

Some changes, on immigration and climate change, for example, can be done by the Trump White House on its own. Others will take Congress, but through the use of budget rules, Republican leaders have already shownthey can effect serious change, such as repealing the Affordable Care Act, without Democrats’ help. They need only the new president’s signature.

Here are some major arenas in which Mr. Trump and Congress have promised action:



On immigration, the changes Mr. Trump will bring will be swift and sweeping. While Mr. Obama had been moving to create openings for undocumented immigrants to attain legal status and increase the number of refugees, Mr. Trump said he would sharply curtail the refugee flow and start a nationwide crackdown on illegal immigration. He will bring a nativist vision to the White House, regarding immigrants warily as competitors for American workers and treating refugees as potential terrorists.

President-elect Trump has promised to start construction right away on a wall across the Southwest border that was the signature proposal and rallying cry of his campaign. That will take money from Congress, but other efforts need only a stroke of the presidential pen.

He said he would quickly cancel a program Mr. Obama put in place by executive action that gave protection from deportation and work permits to about 800,000 undocumented immigrants who came to the United States as children. They will lose jobs and scholarships that allowed many to attend college and start careers, and they will become vulnerable to deportation. Mr. Trump would also permanently cancel a broader protection program for undocumented immigrants that Mr. Obama tried to carry out that was blocked by the courts.

Mr. Trump has said he will initiate a campaign to deport, within the first year of his presidency, what he described as up to two million undocumented immigrants with criminal records, although he has not explained how he reached that estimate. Enforcement operations would ramp up, with widespread raids in communities and workplaces.

Mr. Trump is also likely to expand detention centers along the border for illegal crossers, ending the Obama administration’s practice of releasing many families who have come fleeing violence in Central America so they can seek asylum in immigration courts. Mr. Trump has said he will punish so-called sanctuary cities, which have curbed their cooperation with the immigration authorities, by withholding federal funding.

He has also said he will temporarily halt all Syrian refugees coming to the United States. He has pledged “extreme vetting” for other refugees, including an “ideological certification” by which they would have to show they share American values. He said he would halt immigration from regions of the world that are “compromised by terrorism,” although he has not specified what countries he means.

And he has said he will tighten scrutiny throughout the immigration system, including on foreigners who come to work or to reunite with their families. Although the system already has crippling backlogs, Mr. Trump wants to further reduce legal immigration from its current levels — but he would need the approval of Congress for that.



Climate Change

The shift in Washington comes just as the United States has been assuming a leadership role in combating climate change, and it will almost certainly upend the world’s efforts to curb the impact of global warming.

Mr. Trump has repeatedly denied the science of human-caused climate change, incorrectly calling it “fictional” and saying it was a hoax created by the Chinese. He has said that he will do everything in his power to undo Mr. Obama’s ambitious domestic and international climate change policies.

While some legal and procedural roadblocks would impede a complete gutting Mr. Obama’s existing climate change regulations, Mr. Trump could significantly weaken or slow them. And in sending a message to the rest of the world that the United States does not intend to enact a climate change agenda, the Trump presidency could cause reluctant governments such as those in India and Poland to slow or weaken their efforts to cut planet-warming emissions.

Mr. Trump has vowed to “cancel” the Paris Agreement, the 2015 deal in which nearly every country put forth plans to reduce emissions of carbon dioxide. In fact, it is not possible for a world leader to unilaterally block or undo the accord, nor would it be possible for the United States to legally withdraw from it for at least the first term of a Trump administration. That is because with Mr. Obama and the United Nations secretary general, Ban Ki-moon, urging other countries to ratify the accord rapidly, it entered into legal force last week. Any country wishing to withdraw must wait four years to do so.

But even though the United States, the world’s second-largest source of greenhouse-gas emissions, remains legally bound to the Paris plan for four years, there will be no legal consequences or economic sanctions if Mr. Trump does not follow through with the Obama administration’s commitment. Under the accord, Mr. Obama promised that the United States would cut emissions up to 28 percent from 2005 levels by 2025, largely through a set of Environmental Protection Agency regulations on coal-fired power plants.

Mr. Trump could not immediately block the E.P.A. rules, but over the course of his administration he could ensure that they are weakened or rolled back. Currently, the rules are facing a legal challenge by 28 states and dozens of companies, and the case is expected to go before the Supreme Court as soon as next year. By appointing an industry-friendly Supreme Court justice and failing to argue for the rules, President Trump could weigh the court against them. He could also direct the E.P.A. to rewrite the regulations to be far more lax.

Beyond Mr. Trump’s remarks challenging the science of climate change, he has taken steps to translate those views into policy. The head of environmental policy on his transition team is Myron Ebell, who directs energy and environment policy at the Competitive Enterprise Institute, a conservative think tank, and has gained national prominence for his polarizing skepticism of climate science. Also informing Mr. Trump’s transition team is Michael McKenna, an influential Republican energy lobbyist who has urged Republican lawmakers to deny climate science.



Health Care

Mr. Trump has promised to repeal Mr. Obama’s singular domestic achievement, the Affordable Care Act, and Republicans in Congress have shown the way. Republicans will not have the 60 votes in the Senate needed to pass most major legislation, but through a parliamentary procedure called budget “reconciliation,” they have already done a dry run to gut the existing law without facing a Democratic filibuster.

“Trump can start undoing the law administratively, but most of the action will lie with Congress,” said Chris Jacobs, a conservative health policy analyst who used to work for Republicans on Capitol Hill.

Many provisions of the health law are now deeply embedded in the nation’s health care system. Uprooting them would be a complex political and logistical challenge. Insurers now accept the idea that they cannot deny insurance, or charge higher prices, to people who have been ill.

And Congress may not move instantly to roll back the law without a clear idea of how to replace it — how to insure the 20 million people who have gained coverage under the Affordable Care Act.

Moreover, the Trump administration will not be able to reverse unilaterally the expansion of Medicaid authorized by the health care law. Thirty-one states, including some with Republican governors, have expanded eligibility, with big infusions of federal money. Many of these states would balk at efforts to undo the expansion of Medicaid. The new Medicaid beneficiaries and health care providers, including hospitals, would also fight to preserve the expansion of Medicaid.

So the political environment could change immensely. Congress may spend months on hearings, debate and legislative maneuvering before making radical changes in the health law, on which public opinion has always been deeply divided. An army of lobbyists for doctors, hospitals, consumers, drug makers and insurance companies will descend on Capitol Hill to shape the legislation.

The requirement for most Americans to carry insurance — the “individual mandate,” enforced through tax penalties — is one of the most unpopular provisions of the law and is a prime target for Republicans eager to dismantle it. Republicans could also pass legislation to lift the requirement for larger employers to offer coverage to their workers, under the employer mandate.

It is not certain that Congress would repeal the health law in its entirety, but Mr. Trump and Republicans in Congress could definitely shift direction, reducing the role of government in health insurance markets, cutting back federal regulation and requirements so insurance would cost less and no-frills options could proliferate.

Mr. Trump would encourage the sale of insurance across state lines, in a bid to increase competition. He and the House speaker, Paul D. Ryan of Wisconsin, have said they will convert Medicaid, now an open-ended entitlement, into a block grant, giving each state a lump sum of federal money to provide health care to low-income people.

And Mr. Trump would encourage greater use of health savings accounts and let people take tax deductions for insurance premium payments.



Economic Policy

Republican control of Washington sets the stage for a sweeping shift in economic policy. Mr. Trump has proposed a fairly standard set of conservative prescriptions, such as lower taxes and less regulation, with one notable departure: a promise to reduce trade with other nations.

The centerpiece of Mr. Trump’s plans is a major overhaul of the federal tax code. Mr. Trump has proposed a sharp reduction in the taxation of businesses and a series of changes that would reduce income taxes for most American households. The wealthiest households would see by far the largest reductions in taxes.

The nonpartisan Tax Policy Center estimated that Mr. Trump’s plan would cut federal revenues by $6.2 trillion over the next decade — a significantly larger reduction than the last major round of cuts under President George W. Bush in 2001 and 2003.

The plan reduces tax rates for most kinds of income. The top personal income tax rate, for example, would fall to 33 percent from 39.6 percent. The corporate income tax rate would fall to 15 percent from 35 percent. The resulting loss of revenue would be partly offset by eliminating loopholes and limiting deductions.

Such changes would require legislation, but Mr. Trump’s proposal shares considerable common ground with a plan advanced by House Republicans. And through budget reconciliation, major tax cuts — such as Mr. Bush’s — have been passed with simple majority votes in both chambers.

Mr. Trump and his advisers have insisted that these cuts would not increase the federal debt, in part because they say faster growth would increase tax revenues. Previous tax cuts, however, have not produced anything like the projected increase.

Mr. Trump has also promised to make deep but unspecific cuts in federal spending.

At the same time, however, Mr. Trump has said that he would like to increase federal spending on the military, and on infrastructure like roads and bridges.

An analysis by the nonpartisan Committee for a Responsible Federal Budget estimated that Mr. Trump’s plans would increase the federal debt by $5.3 trillion over the next decade, and raise the ratio of debt to gross domestic product to 105 percent.

Mr. Trump also has promised to reduce federal regulation. Business groups argue that the Obama administration has impeded economic growth by significantly expanding regulation in areas including environmental and worker protections.

He has specifically promised to reverse some new environmental rules, such as the climate change regulations on power plants. Earlier this year, he also proposed the “dismantling” of the Dodd-Frank Act, which overhauled federal regulation of the financial industry in the aftermath of the 2008 financial crisis. The act created the Consumer Financial Protection Bureau, a likely target for Republican legislators.

Mr. Trump also has promised a major shift in trade policy. He says he will increase American manufacturing by reducing imports from China and other nations.

His victory almost certainly seals the demise of the proposed Trans-Pacific Partnership, which the Obama administration negotiated with Japan and other Pacific Rim nations. Mr. Trump also pledged repeatedly to renegotiate or abandon existing trade agreements, notably the North American Free Trade Agreement, or Nafta, which has increased American trade with Canada and Mexico. He also has threatened a variety of sanctions against American companies that move manufacturing jobs overseas, although the legality of such measures is unclear.

Republicans who broadly agree with Mr. Trump on taxes and regulation may have greater reservations about his views on trade. The party has long supported increased trade among nations. But Mr. Trump is the party’s new leader.