Buckle up and brace yourself: Here comes the Trump swerve. After eight years of President Obama there’s a new chief executive entering the Oval Office, and he’s eager to grab the reins of government and steer the United States in a sharply different direction.

The G-forces created by this coming turn might be intense. Seldom in American history have the policy disagreements between a president and his predecessor been so great. Consider that the Affordable Care Act, Mr. Obama’s signature domestic achievement, is high on President Trump’s most endangered list. Mr. Trump is pushing the GOP Congress to repeal the ACA (also known as “Obamacare”) and replace it with something else as soon as possible. And ASAP in this case may mean “days.”

Trump’s likely to sweep away a number of Obama-era business and environmental regulations before inaugural balls get going. The new president’s approach to foreign policy – from Day 1 – promises to be transactional and unilateralist, whereas Obama’s was more alliance-oriented.

The incoming and outgoing presidents seem to get along on a personal level about as well as could be expected for two people with wildly different personalities and political views. So Transition 2017 won’t be awkward on that level. That’s not always so: In 1933, Herbert Hoover and Franklin Delano Roosevelt so despised each other that neither spoke a word as they rode from the White House to the Capitol for F.D.R.’s swearing-in.

AP/File President Herbert Hoover (l.) greets President-elect Franklin D. Roosevelt in 1933 in front of the White House. The two men, who despised each other, didn’t speak during the ride to the Capitol for the swearing-in ceremony.

On policy substance it’s another matter. The change from Obama to Trump might not constitute the U-turn of Hoover to F.D.R., when a tight money approach to the Depression gave way to an expansionist New Deal overnight. But it may be at least as consequential as, say, the switch from Democrat Harry Truman to Republican Dwight Eisenhower in 1953, or from Ike to Democrat John F. Kennedy eight years later. Both those transitions produced real change in how the US approached the world.

One caveat: Trump himself is a big variable. Allegations about his connections to Russia could morph into a story that consumes his time. And it remains unclear how many of his campaign promises should be taken literally. As a novice politician he has little ideological record. How will he adapt to being president, as opposed to running for office? Are his tweets real policy signals or just noise?

And Trump, like some other president-elects before him, will find it’s harder than he thinks to send the government careening off on a new tangent. The federal bureaucracy is skilled at absorbing and diffusing presidential orders. Congress and the courts have a lot to say about what happens in the District of Columbia. “Continuity” may be more Washington’s watchword than “change.”

But Republicans now control both chambers of Congress and the presidency. After eight years of Obama there’s a lot of pent-up demand on the right for GOP-led initiatives.

“I think [Trump] can be transformative not just because of himself but because the conditions are pretty good right now for an aggressive Republican administration,” says Julian Zelizer, a professor of history and public affairs at Princeton University.

Geoff Crimmins/Moscow-Pullman Daily News/AP Republicans at Washington State University in Pullman erected a wall to celebrate Donald Trump’s plans for a barrier along the southern border, which he vows to start building right away.

Like most modern presidents, Trump has already produced a list of things he vows to do in his first 100 days in office. That’s a benchmark that dates back to F.D.R. of course. In his first 100 days F.D.R. pushed 15 major pieces of legislation through Congress. This unprecedented burst of activity established federal insurance of bank deposits and – for the first time – regulated Wall Street. It created US agricultural supports and the Civilian Conservation Corps. It laid the foundation for today’s federal government structure.

Trump’s list is neither that sweeping nor, needless to say, that liberal. It includes potentially big changes nonetheless.

Some aren’t likely to actually take effect – for instance, Trump said in October that he’s going to propose a constitutional amendment to put term limits on Congress. The process to approve that would be lengthy and complex.

But others can be done easily. Trump has promised that he’ll immediately terminate what he terms Obama’s “illegal amnesties” for unauthorized immigrants. That would include the Deferred Action for Childhood Arrivals (DACA) program, which allows unauthorized immigrants brought to the US as kids to stay in the country. He’s said he’ll withdraw from the Trans-Pacific Partnership trade deal Obama officials have worked on for eight years.

Presidents have lots of power to levy tariffs on imports of specific products or from particular countries, as Trump has threatened to do. They have plenty of leeway on foreign policy as well. In that regard Trump has vowed that on Day 1 he’ll go after China by labeling it a currency manipulator – a move that may have little actual effect but will annoy Beijing. And he insists that at the earliest possible moment the US will begin working on Trump’s Great Wall for the southern border (a barrier Obama has called “half-baked”).

The new president has not changed his position in regard to where the funding for this project is coming from.

“Mexico will pay for the wall,” insists Trump’s website still, highlighting the vow in red type.

Pablo Martinez Monsivais/AP President Barack Obama meets with President-elect Donald Trump in the Oval Office of the White House in November.

In some ways obama’s legacy is uniquely vulnerable to reversal or alteration by a new chief executive. That’s because a substantial portion of it is built on a foundation of executive orders and other direct manifestations of presidential power.

Obama did not feel he had much choice. In his first two years in office he enjoyed safe congressional majorities, and he was able to get Obamacare and the Dodd-Frank financial regulation bill through Congress. But in the 2010 midterm elections Democrats lost control of the House and barely held onto the Senate. After that the route through Capitol Hill for Obama’s legislative agenda was pretty much blocked.

So he turned to executive actions. DACA is a case in point: There was no way Obama could get a bill through Congress allowing unauthorized immigrants brought here as youngsters to stay in the US. Instead, he invoked his authority as the boss of federal law enforcement and ordered US prosecutors to use their discretion to leave such people alone. Republicans fumed (and sued) but now that’s a moot point.

“The great thing about executive power is that you can use it with efficiency and speed. The bad thing is that the next president can attack it with the same efficiency and speed,” says Mr. Zelizer.

The Iran nuclear deal is another example of this approach. It’s not a treaty approved by the Senate and enacted into US law. Instead, Obama used his presidential authority to lift Iranian sanctions, his ability to strike political international agreements, and the US vote in the United Nations Security Council to stitch together an accord aimed at curbing Iran’s uranium enrichment activities.

Iranian Presidency Office/AP President Hassan Rouhani (right, c.) and other Iranian leaders listen to the national anthem at the University of Tehran. Whether Donald Trump will pull the US out of the nuclear deal with Iran, as he’s said he wants to, will be a foreign-policy decision watched around the world.

Trump could reverse much of that. During his campaign he vowed he would, saying that his “No. 1 priority is to dismantle the disastrous deal” and extract more concessions from Tehran. Whether he’ll fulfill this promise is an open question – at a recent Christian Science Monitor breakfast the Republican chairman of the Senate Foreign Relations Committee, Sen. Bob Corker of Tennessee, said ripping up the agreement would create an international “crisis” and isn’t likely to happen. But Trump’s unpredictable. The power to act on this issue is in his hands.

Whoever follows Trump into the Oval Office may have the same sort of leeway on other important matters. Heavy reliance on executive actions has become a feature of the modern presidency, says former Senate historian Donald Ritchie. Given the GOP’s current hold on the House and Senate, it might be hard to foresee a day when Trump finds the legislative pathway blocked. But disputes can be intraparty as well as partisan. And the American electorate seems to have become used to punishing the party in power in midterm elections.

“If you look at the last couple of years of every president ... once they lose control of Congress they have got to turn to executive orders if they want to leave a stamp on things,” Mr. Ritchie says.

For now, though, the US government is in united Republican hands. And the congressional GOP is more ideologically unified than ever. The party has moved to the right on many overarching domestic issues such as government spending and taxes. The few moderate Capitol Hill Republicans that remain could caucus in the back of a compact car.

Trump’s constructed a cabinet along similar lines. Even his picks from outside Washington are generally in concert with the congressional GOP’s thinking on domestic issues. Some nominees – such as the secretary-designate of Health and Human Services, Rep. Tom Price – were plucked from Congress itself.

It’s almost certain that Congress will move a tax cut bill, probably shaped along lines long favored by House Speaker Paul Ryan.

Jonathan Ernst/Reuters Donald Trump introduces his wife, Melania, before delivering his remarks at a luncheon with his cabinet members and congressional leaders at Trump International Hotel in Washington.

It’s also virtually certain that lawmakers will repeal Obamacare and enact some sort of GOP health plan in its place. It’s likely that they’ll also try to substantially alter Medicaid and perhaps Medicare.

Trump continues to talk about an infrastructure bill, but that seems to be dropping on his list of priorities. Transition officials say it is no longer “core” and is likely to be addressed only after the Trump administration’s initial burst of action.

On most of these issues Democrats will be involved only on the margins. The US isn’t entering an era of bridge building. It’s continuing a period of partisan divide and rule in government.

Trump won’t transform Washington as a Republican who builds a new coalition with Democrats, says Zelizer. “He’ll transform it as a Republican who might achieve what his Republican predecessors were unable to do in terms of cutting down a lot of government and in some ways using the military more aggressively,” says the Princeton professor.

But there is a wild card here, a known unknown, an X-factor. That’s Donald Trump himself. As president, Trump is singular. Some historians compare him to Andrew Jackson, the first populist president, a man whose supporters the elite felt to be uncouth.

But Jackson had been a state governor and a general. Trump’s the only US chief executive in history who has never held political office or been a military officer. He’s the first to use social media to attack his adversaries. His blunt campaign style has been unique and refreshing to some, and horrifyingly transgressive to others.

What will he actually do? That remains an open question. Some of his recent tweets, if serious, have vast policy implications. In December he tweeted that the US “must greatly strengthen and expand its nuclear capability,” for instance. That implies he will propose a large new atomic weapons program, perhaps launching a new arms race. Unless the tweet was just a random musing thought, the kind all presidents probably have but haven’t previously made public.

Trump’s also tweeted that flag burners should face loss of US citizenship or some other sort of legal punishment. That would likely run afoul of First Amendment protections. Is he serious? He often complains about unfair media treatment and labels particular newspapers or TV programs “failing.” He’s talked about loosening libel laws. Will he use legal powers to go after the press?

The Trump Cabinet is conservative – predictably so, says David Greenberg, a professor of history and journalism at Rutgers University. With a few exceptions, its members are the same sort of people any of the Republican presidential candidates would have picked.

But Trump has also selected for his inner circle some “wild outsiders” who want to blow up the current political system, says Mr. Greenberg, such as former Breitbart News chair Steve Bannon. In that sense the 2017 transition might be similar to the 1969 handover of power from Lyndon Johnson to Richard Nixon.

Nixon picked a conservative cabinet but generally ignored it and dealt mostly with a favored few aides, such as Chief of Staff H.R. Haldeman. Nixon’s temperament wasn’t so much conservative as authoritarian, Greenberg adds. “The worry most people have about Trump is along those lines,” he says. “People worry he will govern the way he has campaigned ... [that] he will show the same contempt for norms that he has throughout the campaign.”

In that sense the transition from Obama to Trump will involve more than just a change in policies, says the Rutgers historian.

It’s also possible that Trump is less a breaker of Washington’s crockery and more a rookie politician who hasn’t yet figured out how to translate his business experience into presidential leadership.

If that’s the case, the best comparison might not be Nixon, but Jimmy Carter. President Carter was an outsider who knew little about how to get things done in the nation’s capital. To some extent, he never did figure it out. Carter, a former Georgia governor, treated Congress as if it were the Georgia state legislature – something you could go around via direct appeals to voters. That just did not work on the national level after he won the presidency.

“He had huge Democratic majorities and didn’t make the most of [them],” says Ritchie, the former Senate historian.

That example would foreshadow friction between Trump and the congressional Republican majority. There have been some examples of that in the pre-inaugural period: Trump objected to the timing of the House GOP effort to downgrade the independent Office of Congressional Ethics. Many Republican lawmakers have questioned Trump’s developing geopolitical “bromance” with Russian leader Vladimir Putin.

For now, the GOP congressional leadership appears willing to accommodate Trump’s tweets and other eccentricities to get his presidential signature on long-sought conservative legislation. But this accommodation has shallow roots. Remember that back in June Mr. Ryan called Trump’s complaints about a Hispanic judge a “textbook definition of a racist comment.”

“Who is going to check [Trump]? It might be his own party in Congress,” says former House historian Raymond Smock, director of the Robert C. Byrd Center for Congressional History and Education.

Shannon Stapleton/Reuters Donald Trump smiles as he is applauded by his son Eric Trump (l.), daughter Ivanka Trump, and son Donald Trump Jr. (r.) at a recent press conference at Trump Tower in New York.

Reality might checkTrump’s policy ambitions, too.

The US government isn’t actually a Ford Mustang (American made!) that a new president can whip into a tight slide and turn. It’s more like a container ship, a behemoth of the seas that has lots of inertia and takes miles to stop. Not exactly maneuverable.

All presidents come into office wanting to make an immediate impact. But that is harder than it looks.

“There is a lot more continuity than people think,” says George Edwards III, a presidential expert and distinguished professor of political science at Texas A&M University.

For one thing, the bureaucracy resists. This is a matter of procedure as much as obstinacy. It’s easy for a new president to sign an executive order undoing a predecessor’s executive order. But will there be a new regulation replacing an old one? Does it need to be published in the Federal Register for public comment? What’s its effect on the budget? And so forth.

For another, existing laws and/or regulations usually develop constituencies. Take Obamacare. Republicans might want to go back to the era prior to the ACA, but doing so would involve taking health insurance away from millions of Americans. It would mean insurers could again deny coverage to people with preexisting conditions. Whatever its faults, the Obama health effort has moved the goal posts on the issue. Trump and the GOP need to take that into account.

Mike Segar/Reuters/File A website shows the sign-up page for the Affordable Care Act (‘Obamacare’), something that Mr. Trump wants to repeal immediately.

Congressional Republicans are struggling with that right now as they try to put together an Obamacare replacement as quickly as possible. “You might make some adjustments, but they’ll provide health care to those millions of people,” Mr. Edwards predicts.

Nor do American interests in foreign policy change just because the nation held an election. A new president, taking office, often finds that there are good reasons the US has taken the international positions it has. The Iran deal might be a good example of this. If Trump rips it up, what will he do next? Lots of other nations had a say in its creation and aren’t eager to return to what existed before. That will greatly lessen US leverage. Meanwhile, Iran will demand changes of its own.

North Korea remains one of the biggest problems facing US diplomacy. Trump has already vowed that North Korean development of an intercontinental ballistic missile “won’t happen.” But as it happens, China is a huge influence on North Korea. It’s Pyongyang’s biggest neighbor and only friend. Will plunging into a trade war with Beijing help the US control Kim Jong-un?

Finally, presidential honeymoons are short. Trump is working with congressional majorities, which is good news for him, but he is also not particularly popular with voters for an incoming president, which is bad news. That will lessen his ability to get difficult things through Capitol Hill.

“Once he does things that really irritate people and there is pushback – ‘here is the guy who wants to make the air dirty’ or ‘business leaders say this will be bad for jobs’ – he is going to be even less popular,” says Edwards.

All presidential transitions are uncertain. The new president and the new executive branch team are untested. Other countries (Russia?) may see the transition period as a time to prod and test the US. Others (Israel?) may see it as an opportunity to get on better terms with the American administration. “But this one seems more uncertain. Trump has never held public office and he doesn’t have a long history of opinions in public policies,” says David Clinton, chair of the political science department at Baylor University.

Stevo Vasiljevic/Reuters STEVO VASILJEVIC/REUTERS Mr. Trump’s charitable comments about Russian President Vladimir Putin – the two men are shown here on a billboard in Montenegro – continue to stir a row in Washington.

Today’s situation might be comparable when Eisenhower took over from Truman in 1953, according to Mr. Clinton. Eisenhower had vowed during the campaign to go to Korea, then the theater of a shooting war. He hinted there would be a dramatic change in US strategy. He also instituted Project Solarium, a famous discussion in which three groups argued for three different US grand strategies in the developing confrontation with the Soviet Union.

In the end, Ike made tweaks in the US approach in these areas, but they were minimal. Containment remained the White House watchword for the cold war. “As it turns out, there wasn’t as much change as people thought there might be,” says Clinton.

That’s the way it has often been with transitions, he says. New presidents discovered that the US ship of state has so much inertia, and takes so much energy to change course, that it is best to single out priorities and work hardest on those.

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“They just focused on a few issues, on a few things they thought they could handle. And that’s what happens with most presidents,” Clinton says.

Contributor Gail Russell Chaddock and staff writer Story Hinckley contributed to this report.