Donald Trump is becoming a president without a party.

From the start of his unconventional campaign two years ago, Trump has been more of a populist than a Republican, from his combative style to his protectionist stance on trade. His ability to reach voters drawn by his personal appeal rather than his party affiliation has been a source of political strength and possibility in a nation where allegiance to both Republicans and Democrats has eroded.

But the most disruptive week of a disruptive presidency is testing whether other elected Republicans will continue to back him up, and whether he can govern if they don't.

Trump, whose tenure already has worn out use of the word "unprecedented," is ignoring some lessons of history about what presidents need to do to get things done.

"He was elected by a minority vote and, as the polls show, continues to cultivate a populist base that is not a majority," says Harvard professor Joseph Nye, author of Presidential Leadership and the Creation of the American Era. "In contrast, George W. Bush, who also won the presidency without a popular majority vote, moved to the center and reached across the aisle to Senator (Edward) Kennedy and others. Trump has not reached across the aisle, and he has also attacked important senators in his own party.

"This makes it difficult to govern in a country where the constitutional separation of powers requires the executive and legislative branches to cooperate."

In recent days, the GOP's governing coalition has been at risk of unraveling:

• The stunning collapse early Friday morning of Senate efforts to repeal the Affordable Care Act — a core campaign promise by Trump in 2016 and by congressional Republicans for the past seven years — underscored GOP divisions that neither the president nor congressional leaders were able to bridge. Afterward, Trump unleashed a tweetstorm of attacks, threats and demands on his fellow Republicans.

• The messy ouster of White House chief of staff Reince Priebus announced a few hours later continued what effectively has been a purge of presidential aides with the closest ties to the GOP establishment, among them former press secretary Sean Spicer, former deputy chief of staff Katie Walsh and former communications director Michael Dubke.

• The public humiliation of Attorney General Jeff Sessions, who was serving his fourth term as an Alabama senator when he resigned to join Trump's Cabinet, prompted Senate Republicans to circle the wagons around their former colleague. Judiciary Chairman Chuck Grassley warned the White House — on Twitter, of course — that the committee wouldn't have time to hold confirmation hearings on a replacement this year if the president fired Sessions.

• The easy passage of a bill tightening Russian sanctions reflected the determination by Republicans as well as Democrats to tie the president's hands when it comes to dealing with Moscow. Passed by veto-proof margins last week — the Senate vote Thursday was 98-2 — the White House had little choice but to say Trump would sign it.

When Trump was elected last November by carrying such traditionally Democratic states as Wisconsin, Michigan and Pennsylvania, some analysts argued he had an opportunity to tap broad dissatisfaction with politics-as-usual and forge a sort of Party of Trump. Some presidents have managed to govern with a certain partisan independence. After Democrats were rebuked by voters in the 1994 midterm elections, then-president Bill Clinton adopted a strategy of "triangulation" that distanced himself from congressional Democrats and cut deals with Republicans on a welfare overhaul and deficit reduction.

But Trump made no effort to reach across the aisle when he took office on the opening health care battle or other issues, and the reality of the polarization that has divided the capital into warring tribes might have made it impossible to achieve even if he had.

"Polarization in Congress is so high that presidents have no margin of error to lose members of their own party, like tightrope-walking with no net," says political scientist Brandon Rottinghaus of the University of Houston. "Without cementing party solidarity and no hope for crossover votes from the other party, a polarized president cannot legislate."

He cautioned: "A president without a party in polarized Washington is in danger of being irrelevant."

Consider the debate over what to do next on health care.

Trump says fight.

"Unless the Republican Senators are total quitters, Repeal & Replace is not dead!" he tweeted Saturday. "Demand another vote before voting on any other bill!"

He coupled that with a threat to stop paying the subsidies to insurance companies that help control premium costs for low-income Americans and others on the Obamacare exchanges, which are also used by Congress: "If a new HealthCare Bill is not approved quickly, BAILOUTS for Insurance Companies and BAILOUTS for Members of Congress will end very soon!"

But Senate Majority Leader Mitch McConnell, in a speech on the Senate floor Friday, said it was "time to move on." The Kentucky Republican, who takes the prerogatives of the Senate seriously, hasn't publicly responded to a string of caustic presidential tweets over the weekend demanding that the Senate abolish the legislative filibuster.

"Republicans in the Senate will NEVER win if they don't go to a 51 vote majority NOW," Trump tweeted. "They look like fools and are just wasting time."

In fact, the measure to repeal the Affordable Care Act wasn't blocked by a filibuster. Republicans used a legislative maneuver that required only a 51-vote majority to pass; they managed to get just 49. McConnell has said in the past he wouldn't end the legislative filibuster — aware, perhaps, that Republicans are more than likely to find themselves in the minority down the road.

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Congressional Republicans seem increasingly willing to buck the White House. Both the Senate Intelligence Committee and the House Intelligence Committee last week questioned White House adviser and presidential son-in-law Jared Kushner about the Russia allegations that Trump denounces as a hoax and political witch hunt.

And Alaska Sen. Lisa Murkowski appeared unconcerned when she confirmed on Thursday that Interior Secretary Ryan Zinke had called her to threaten retribution against The Last Frontier if she persisted in voting against the Republican proposal to scale back the Affordable Care Act.

While Trump remains a formidable political force in some states and congressional districts, his national standing continues to hit historic lows for a president early in his term. In the latest USA TODAY/Suffolk University poll, 42% of those surveyed approved of the job Trump was doing as president; 53% disapproved. His favorable-unfavorable rating also was underwater by double digits, 40%-55%.

That said, his favorability standing was still better than the ratings for the Democratic Party (35%-51%) or Republican Party (32%-55%).

Arizona Sen. Jeff Flake, one of the two most vulnerable GOP senators up for re-election next year, defended efforts by Republicans to push back to Trump. "There are times when you have to stand up and say, 'I'm sorry. This is wrong,'" Flake said Sunday on CBS' Face the Nation. "There are truths that are self-evident. And you've got to stand up and call — whether it's the White House or other elected officials — to task when they're not doing what they should."

That was not the message from Corey Lewandowski, at one point Trump's campaign manager. He offered some free advice to incoming White House chief of staff John Kelly, a retired four-star Marine Corps general.