NEW YORK — President-elect Donald Trump is set to take office under immense pressure to quickly deliver on a list of audacious campaign promises that served as the cornerstone of his bid to disrupt Washington and undo pieces of President Obama’s agenda.

Some of Trump’s most dramatic undertakings – such as canceling Obama’s “illegal” executive actions – can be done in his first hours as president. Other priorities, such as repealing and replacing the Affordable Care Act or building a wall on the U.S.-Mexico border, will require the approval of Congress, which will be controlled by Republicans but could still squabble over details. And other efforts could run into political or legal obstacles that may be difficult to overcome.

For Trump, the transition from proposing drastic changes on the campaign trail to trying to navigate the complex gears of government to implement them will serve as a jarring early test of his tenure in the White House.

In the short term, his victory means that at some point next year, the nine-member U.S. Supreme Court will be restored to full capacity with a majority of Republican-appointed justices. And it likely changes the court’s docket, as well: With a stroke of the pen, the new president could cancel Obama’s regulations regarding the environment and immigrants, issues that have preoccupied the justices in recent terms.

Besides replacing Justice Antonin Scalia, who died in February, Trump may get the chance to replace liberal justices and move the court to the right for generations.

Two of the court’s liberals, justices Ruth Bader Ginsburg and Stephen Breyer, are 83 and 78, respectively. Moderate conservative Justice Anthony Kennedy is 80.

CONFLICTING POLICY PROPOSALS

After a campaign of bombastic sound bites and often contradictory policy prescriptions, Trump’s plans for dealing with issues such as terrorism, Russian aggression and multiple shooting wars in the Middle East remain unclear.

He has simultaneously called for increased military strength and more forceful American leadership, while also speaking of stepping back from U.S. responsibilities as the free world’s primary protector. He has invited China to invade North Korea and “solve that problem,” but said he would host North Korean leader Kim Jong Un at the White House.

He has said he would renegotiate the Iran nuclear deal, and then called for strict enforcement of the existing agreement. His plan to combat the Islamic State, Trump said during the campaign, was a secret.

With little clarity on much of what he intends to do, the best initial indicator of Trump’s approach may be whom he chooses for his national security team. Many of the names floated so far strike fear in the hearts of mainstream Republicans as well as Democrats, while others instill a sense of reassurance.

Perhaps more than any other area of policy, the universe of potential picks for national security Cabinet positions, agency heads, deputies and those below is limited by the large number of Republican establishment figures in the field who said early and often during the campaign that they would never support a Trump candidacy or work in a Trump administration.

Among the rumored candidates for secretary of state, former House speaker Newt Gingrich, R-Georgia, and former U.N. ambassador John Bolton – both outspoken Trump supporters – are viewed as anathema by many current diplomats and as loose cannons even by many of their fellow Republicans. Sen. Bob Corker, R-Tennessee, the chairman of the Senate Foreign Relations Committee, is seen as the more mainstream candidate.

POTENTIAL FIRST-DAY DECISIONS

Two of Trump’s campaign ideas could probably be realized as early as his first day in office: scrapping executive orders issued by Obama – including those that shielded from deportation some immigrants who are here illegally – and appointing a special prosecutor to investigate vanquished Democratic opponent Hillary Clinton.

The former is a common tactic for new presidents whose predecessors belong to the opposing political party. Obama signed an executive order ending a ban on federal funding for embryonic stem cell research imposed by George W. Bush early in his first term.

The latter would be a political risk. By going after the opponent he just defeated, Trump could imperil his chances of broadening his appeal to the millions of Americans who did not vote for him.

“He certainly could do it, but it could have a major, devastating impact on her and would create a very bad precedent like we see in Third World countries” where election winners often imprison their rivals, said John Banzhaf III, professor of public interest law at George Washington University and a scholar on administration law.

Repealing the ACA would take an act of Congress, as would levying some types of tariffs on corporations that move operations overseas, ending regulations that limit pollution and coal production, getting rid of gun-free school zones, and renegotiating the Iran nuclear deal. Trump would have the authority to renegotiate trade deals such as the North American Free Trade Agreement that he has long railed against – and to withdraw with six months’ notice if he wished – but such a move could be catastrophic for stock markets and the economy.

For years, Republicans on Capitol Hill have tried to shred Obama’s signature health care law, only to be stopped by the president. But after holding their majorities, they are finally in position to make it happen in conjunction with the new Republican president.

While Trump spent countless hours on the campaign trail assailing the law, he has yet to fully articulate what he would offer as an alternative. Trump said that he would encourage health savings accounts, allow insurers to sell policies across state lines and convert Medicaid from an entitlement program to a block grant to states – ideas long favored by Republican conservatives. As for the rest of Trump’s plan, he has assured supporters that it will be something “terrific” that is “so much better, so much better, so much better.”

Building a wall along the nation’s southern border would require Congress to commit hundreds of millions of dollars to make it happen. Trump also has no power to force Mexico to pay for it, as he has repeatedly promised to do, although he could pressure the Mexican government with threats to limit trade or drug-related law enforcement activities.

FROM RHETORIC TO REALITY

History shows it can be hard to fulfill vows that sounded easier to make in front of cheering crowds on the campaign trail. For example, Obama immediately issued an executive order closing the military prison for suspected terrorists at Guantanamo Bay in Cuba, but members of both parties in Congress voted to block its closure. In the final months of Obama’s presidency, the prison remains open.

Heavily surveilling mosques in America in the way Trump has advocated would require courts to reinterpret constitutional protections and rights. And if he wants to follow through with his proposed ban on most Muslims from entering the country – which he stopped talking about in the final months of the campaign even as it remained on his campaign website – would immediately be challenged in court as either unconstitutional or against current law, legal experts said.

But Trump would probably have the ability to ban a narrower group of Muslims living in certain parts of some countries controlled by Islamic State terrorists because the immigration statutes afford some discretion on national security grounds.

Another category of Trump’s promises amounts to changing the priorities of agencies. That would require getting Congress to allocate more money for programs already underway, such as deporting illegal immigrants with criminal records and broader promises to “fix the Veterans Administration” and “start taking care of our military.”

In the area of national security, where presidents traditionally have more leeway, Trump could order his top military chiefs to come up with a plan within 30 days to stop the Islamic State, as he has promised. But if the plan would require the deployment of troops to Iraq and possibly an invasion of Syria, a war-weary Congress eventually would need to sign off and agree to fund a large and open-ended increase in military spending.

BIG TAX CUTS, REVIVING WATERBOARDING

Even if the plan were to rely on the CIA’s covert action authority and be carried out in secret, Trump would face limits on what he could do and what the professionals at the CIA, the National Security Agency and other intelligence agencies might be willing to do.

For example, if he tried to order the use of torture against terrorists, as he has pledged, Trump would immediately run into legal impediments and a buzzsaw of resistance from CIA lawyers and operators still smarting from a decade of revelations and public criticism over waterboarding and secret prisons, intelligence officials said.

Trump would have to persuade Congress to overturn the ban on waterboarding instituted by George W. Bush in 2006 and the congressional ban on any interrogations that go beyond the restrictive ones outlined in the Army Field Manual.

At his final campaign rally in Grand Rapids, Michigan, on Monday, Trump said he would put in place “the largest tax cut since Ronald Reagan” and offered less specific ideas such as eliminating “every unnecessary, job-killing regulation,” protecting religious liberty and rebuilding the military and law enforcement.

“Just think about what we can accomplish in the first 100 days of a Trump administration,” he told the crowd.