The question, as President Donald Trump and GOP leaders celebrate their generational overhaul of the tax code, is how long it will last.

In unseasonable sunshine on the White House South Portico on Wednesday, Trump and the GOP savored a moment of togetherness and triumph, at last validating their monopoly on Washington power.

"We just got together and we would work very hard ... it was a lot of fun. It's always a lot of fun when you win," Trump said, turning to House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell, the partners in his uneasy marriage of convenience between the GOP's insurgent and establishment wings.

"All friends. I mean, I look at these people: It's like we're warriors together," Trump said.

Wednesday's picture of a President perfectly in sync with powerful congressional leaders, getting big things done, was what Republicans expected to see when they captured control of Congress and the White House a year ago and foresaw a sustained period of conservative rule.

Yet such moments have been fleeting in a year in which the President has squandered precious first term power and Capitol Hill leaders botched attempts to repeal and replace Obamacare amid a civil war raging within the GOP.

The big hitters at Trump's victory party, in between lavishing one another with towering praise — Ryan, for instance, hailed the President's "exquisite" leadership — vowed that the big tax win was a turning point.

"We're just getting started," said Vice President Mike Pence. GOP House Majority Leader Kevin McCarthy added: "This is a big day for America. This is America's comeback." Sen. Orrin Hatch of Utah, outdoing his peers in his praise of Trump, predicted the newly united GOP would "make this the greatest presidency that we've seen not only in generations, but maybe ever."

Strangely, the master of hyperbole, Trump himself, seemed a little less sure that the tax bill would unleash a torrent of Republican success.

"I don't know if we will have bigger moments, but we hope to. We're going to try," he said.

When the party finally dies down, Republicans know that they have a job on their hands to sell a tax reform drive that was vital to retaining the faith of their own voters ahead of midterm elections next year but also is deeply unpopular.

A CNN poll this week found that only a third of Americans backed the bill, and a majority oppose it. If the new law does not trigger the promised era of prosperity and job growth, it's weighting towards the wealthiest Americans and corporations could become a big political loser and spoil the appetite of GOP lawmakers for more risky votes next year.

Trump and congressional leaders also know that on other big issues on their plate next year — welfare reform, immigration and infrastructure spending, for instance — the kind of synergy on display in the tax reform drive, a project written in the Republican Party's DNA, will be far more difficult to achieve.

It's possible that in his moment of triumph, Trump could examine how he helped marshal Republican support on tax reform and how he failed, for example, in the acrimonious Obamacare debate, and modify his behavior.

However, those expecting Trump to change, and to avoid the politically costly outbursts and days-long controversies on the nation's most divisive social and cultural issues, have been disappointed many times before.

That's one reason why it's too early to assume that the tax reform effort is a template for future Republican legislative efforts.

Another is that while Republicans generally agree on the need to lower taxes, there is no similar unanimity of purpose on issues like health care, immigration or welfare, especially in the increased intensity of a midterm election campaign that will be raging next year.

Even the current Republican good mood is facing an immediate test, since Trump and GOP leaders now have to honor promises they made to win the votes of wavering Republicans.

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The one moment of discord at the White House ceremony came when Trump called out Sen. Susan Collins of Maine, who was not there.

Collins backed the legislation after being told by McConnell that she would get a vote on two Obamacare market stabilization bills before the end of the year. That is not now going to happen, raising questions of whether the White House can keep the commitments that knitted the fragile GOP coalition together.

White House legislative director Marc Short told Wolf Blitzer on CNN's "The Situation Room" Wednesday that Trump would be as good as his word, saying that "if the bill gets to his desk, he will sign that."

But if the White House cannot deliver, it may find it harder to persuade Republicans to line up behind its initiatives, especially on efforts that could cause supporters in Congress to pay a delayed political price.

Another GOP senator and frequent Trump critic who had wavered on the tax package, Arizona's Jeff Flake, said he had won a commitment on a vote to shield from deportation hundreds of thousands of undocumented immigrants brought into the United States as children.

The issue is deeply problematic for many Republican lawmakers, especially in districts that bought into Trump's immigration message in last year's election, raising doubts over whether a legislative fix is feasible and whether the promise to Flake will be honored.

But in his interview with Blitzer, Short said: "We are doing everything we can to make it happen."

Another Trump critic, Republican Sen. Bob Corker of Tennessee, endured a torrent of criticism for withdrawing his opposition to the tax bill after earlier warning he would not vote for a measure that swelled the budget deficit -- offering a hint of the intense pressure that GOP leaders will face as they try to keep their majority together.

And with their reduced one-vote majority, which will be in place when newly elected Democrat Doug Jones of Alabama arrives in Washington to be seated in the Senate next month, GOP leaders cannot afford to lose a single vote as they seek to ensure that the tax bill is just the start of a new period of productivity in Republican-led Washington.