A Major Challenge for Trump’s Agenda: A Mercurial Coach Calling the Plays

House Republicans are decamping to Mar-a-Lago this weekend to huddle with President-elect Donald J. Trump and plot their strategy as they toil to unite around his behemoth of a legislative agenda.

But before they can do so, they need Mr. Trump to weigh in on an issue they have been agonizing over for weeks: how to structure and sequence his long list of priorities so it can make it through Congress.

They have been anxiously waiting for Mr. Trump to definitively declare his preference so they can set to work producing either one big bill or two more modest ones.

So far, he has mostly equivocated and sent mixed signals.

“Whatever it is — doesn’t matter,” Mr. Trump told reporters at the Capitol on Wednesday before meeting with Senate Republicans. “We’re going to get the result and we’re going to make America great again.”

It was a familiar reprise for Republican leaders who were around during Mr. Trump’s first term to experience how his changing moods, shifting priorities and flashes of rage could sap their leverage and derail even their best-laid plans. And it underscored a steep challenge they face as they try to put together consequential legislation aimed at cutting taxes, slashing spending and cracking down on immigration that he has demanded.

During his first term, Mr. Trump functioned less as a coach calling plays and more as a cantankerous owner demanding that his team throw out the entire playbook in the fourth quarter. In 2018, he threatened to veto a spending bill passed by a Republican-controlled Congress, only to relent. Months later, he threatened to veto an immigration bill backed by the speaker at the time, Paul D. Ryan, only to have his aides walk the comments back. In 2020, he said he would veto a $900 billion Covid relief and government funding bill, but signed it days later.

He played a similar role last month, when he blew up a bipartisan funding deal negotiated by Speaker Mike Johnson three days before the shutdown deadline — and introduced a new demand to raise the debt limit that caught Republican leaders off guard and caused a revolt within his own party.

Republicans on Capitol Hill who lived through Mr. Trump’s first term say it is a fact of legislative life that they have become accustomed to. Some of his fiercest allies argue it is his prerogative to decide to change course.

“He could change his mind,” said Representative Jodey C. Arrington of Texas, the chairman of the House Budget Committee. “And I won’t fault him when he does and if he does when that happens.”

But they also privately concede that it makes it exceedingly difficult to negotiate politically tricky deals on issues that would be troublesome even with a predictable president guaranteed to stay on board. It means that Republican lawmakers never know whether they can count on cover from Mr. Trump and his MAGA army of influencers and activists or should instead be bracing for a torrent of abuse or even a primary challenge.

For now, it has vastly complicated the task of Republicans who are laboring to sort through the dozens of complicated policy choices they will have to execute to push through Mr. Trump’s legislative agenda.

G.O.P. leaders say Mr. Trump’s imprimatur on whatever legislation they ultimately fast-track through Congress will be crucial to ensuring its passage at a time when Republicans are working with slim majorities in both the House and the Senate. Mr. Johnson has said that one of the benefits of one large bill is that it would increase the pressure on Republicans to vote yes or be blamed for tanking Mr. Trump’s entire agenda.

“No one’s going to love every element of a large package like that,” Mr. Johnson said in an interview on Fox last weekend. “But there will be enough elements in there to pull everyone along. They’ll be able to justify not getting all of their preferences on some of their big issues, because there’ll be so many other very important pieces to that one piece of legislation.”

For that strategy to work, however, Mr. Trump will need to stay on message as the bill churns through Congress — a feat that some Republicans say they have privately warned lawmakers not to expect.

Part of the problem is that the television-obsessed president sometimes changes his mind about a policy measure after seeing a lawmaker or talking head make a compelling point on the air, or after being forcefully lobbied in person.

In 2017, after Mr. Trump threatened to veto a large spending bill to prevent a government shutdown, Mr. Ryan dashed to the White House to plead with him to support the legislation. Two days later, on the eve of the shutdown deadline, Mr. Trump again floated the idea of vetoing it on Twitter, after seeing an episode of “Fox & Friends” deriding the bill, The Washington Post reported.

Mr. Ryan, who had by then left Washington to return to his district in Wisconsin for a recess work period, then called Mr. Trump and persuaded him to reverse course — again.