WASHINGTON — Almost every new president has hit Washington with a sense of swagger and talk of barnstorming the town in the first 100 days.

Then they met the United States Senate.

Now, it’s President-elect Donald Trump’s turn to acquaint himself with a place of strange rules and rituals, amazing ego and ambition, where friends become enemies in a matter of hours – and where many previous administrations have perished.

As the 115th Congress prepares to be sworn into office Tuesday, talk of fundamentally transforming Washington has filled the air. The emerging policy agendas of Trump and House Speaker Paul D. Ryan, R-Wis., – whose on-again, off-again relationship dominated 2016 – seem to be meshing on key items such as overhauling the Affordable Care Act and revoking President Barack Obama’s executive orders on business regulation.

But the new president’s early months in office are likely to rise or fall in Majority Leader Mitch McConnell’s Senate, where a narrow Republican majority guarantees the confirmation of most nominees and an ability to set the agenda – but little else.

The minority Democrats are afforded enough rights to turn confirmation hearings for Trump’s Cabinet picks into a referendum on the president-elect’s own policy views and qualifications to lead. By demanding scrutiny of nominees’ personal finances, they can revive questions about Trump’s own holdings.

The Senate’s 60-vote threshold for clearing a filibuster on most legislation means that Trump will have to reckon with Democrats often. Sen. Charles E. Schumer, D-N.Y., plans to drive home this point in his maiden speech as the new minority leader.

“The Senate has a rich, bipartisan tradition of being a constitutional check on presidents of both parties,” Schumer will say Tuesday, according to an advance copy of his planned remarks. “Many in this body have long observed that in America, we are a nation of laws, not men. That sacred constitutional duty of holding the president accountable to the law must continue.”

THE McCONNELL FACTOR

The last time one party controlled the Oval Office and all of Capitol Hill, Obama and Democrats pushed a mostly partisan agenda that Republicans resisted and then used against them in subsequent elections.

McConnell, R-Ky., enjoyed a good relationship with Trump throughout the campaign. Unlike Ryan, he kept his doubts about Trump’s campaign style mostly to himself, and he has forged an early bond with Vice President-elect Mike Pence.

Yet, less than 24 hours after Trump’s stunning presidential victory, McConnell tried to set expectations on his own terms. His early signals have been about bringing along Democrats rather than changing Senate rules in a unilateral power play.

“We’ve been given a temporary lease on power, if you will. And I think we need to use it responsibly,” McConnell told reporters Nov. 9. “I think what the American people are looking for is results. And to get results in the Senate, as all of you know, it requires some Democratic participation and cooperation.”

Democrats are only one piece of Trump’s Senate problem. If past is prologue – and it almost always is in the Senate – Republicans will betray him at key moments and cause big headaches early in his presidency.

Senators have their own egos and their own agendas, and many of them see a president every time they look in the mirror. This has already been on display in early statements from Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., in joining Schumer to call for aggressive investigations of Russian meddling in the 2016 elections.

McCain is getting sworn in Tuesday to a seventh term after winning re-election in November by a wider margin in Arizona than Trump received over Hillary Clinton. That gives McCain – already a self-proclaimed maverick – extra political freedom because he wasn’t swept into office on the new president’s coattails. Sens. Marco Rubio, R-Fla., Rob Portman, R-Ohio, Ron Johnson, R-Wis., and Charles Grassley, R-Iowa, all won re-election by bigger margins than Trump did in winning their states.

Rubio has already rattled the Trump transition by expressing doubt about Rex Tillerson, the ExxonMobil CEO with close ties to Russia who is Trump’s pick for secretary of state. The transition responded with a concerted effort to court Rubio, knowing that just one Republican defection at the committee level could imperil the nomination.

Such maneuvers are likely to be regularly necessary for Trump, who, as a novice politician, is not steeped in the quirks or pretensions of the Senate.

His closest ally in the chamber has been Sen. Jeff Sessions, R-Ala., who is now his nominee for attorney general. Sessions is generally well-liked on both sides of the aisle but has always been considered an ideological outlier, more successful at blocking something than orchestrating its passage.

RECENT TRACK RECORD

Recent history is replete with examples of new presidents running into the gantlet of the Senate. In 1993, Bill Clinton’s administration watched as several Cabinet picks withdrew in disgrace despite a large Democratic majority in the Senate. The rocky relationship hit an even lower point the following year in an open feud with Democrats over his bid to reshape health laws.

In 2001 George W. Bush’s administration watched his proposed tax cut shrink by hundreds of billions of dollars to meet demands of moderate Senate Republicans – and watched his own party lose the majority when then-Sen. Jim Jeffords of Vermont angrily switched caucuses and handed Democrats a 51-49 majority.

In 2009 Obama, a former senator, seemed poised to dominate the body, taking from its ranks his vice president, Joe Biden, his secretary of state, Clinton, and interior secretary, Ken Salazar. But for him, too, things fell apart quickly.

Nominees withdrew amid leaks from Democratic-controlled committees. The economic stimulus package shrank by hundreds of billions of dollars because moderate Senate Democrats fought big spending. And the Affordable Care Act’s final negotiations all centered around meeting demands of moderates to clear a Senate filibuster.

McConnell hopes to avoid such mistakes, but it’s unclear whether conservative activists will appreciate his restraint at a time when Republicans control all the keys to power in Washington.

“It’s always a mistake to misread your mandate. And frequently new majorities think it’s going to be forever,” McConnell said a day after Trump’s victory. “Nothing is forever in this country. We have an election every two years right on schedule. We have had since 1788. And so I don’t think we should act as if we’re going to be in the majority forever.”