The question of whether the president-elect would back the speaker for another term has hung over Republicans, especially after recent tensions. But the endorsement does not lock up the job for Mr. Johnson.

President-elect Donald J. Trump endorsed Speaker Mike Johnson for another term in his post on Monday, moving to shore up the fortunes of a leader whose fate he threw into question this month when he sank a bipartisan spending deal Mr. Johnson had struck to avert a government shutdown.

The announcement from Mr. Trump on his website, Truth Social, ended days of private discussions by the president-elect and his allies about whether to try to save Mr. Johnson or find another candidate, as some conservatives have been agitating for. It followed a concerted, monthslong effort by Mr. Johnson to ingratiate himself with the president-elect in hopes of winning his backing and averting another messy fight for the speakership at the start of the 119th Congress on Friday.

But it was not clear whether Mr. Trump’s stamp of approval would guarantee Mr. Johnson, a Louisiana Republican, a smooth road to re-election, given the antipathy he has drawn from some right-wing lawmakers who have recently shown their willingness to buck Mr. Trump. Given the very narrow margin of the incoming House majority, the loss of just a few votes could stymie Mr. Johnson.

One Republican lawmaker, Representative Thomas Massie of Kentucky, has already pledged to oppose him, and others have said their support is not a certainty.

The House is set to choose a speaker as the new Congress convenes, just three days before Mr. Trump’s Electoral College victory is scheduled to be certified by a joint session of Congress. Mr. Johnson must cobble together a majority in the 435-member chamber to keep his job, a feat that his predecessor Kevin McCarthy struggled to accomplish even though he, too, had Mr. Trump’s endorsement at the time.

Failure to have a speaker in place by Jan. 6 could delay the certification process and focus attention on the deep divisions within the narrow House Republican majority. It also could slow what Mr. Trump and G.O.P. leaders had hoped would be a rapid start to legislative business in the new Congress to begin enacting the president-elect’s ambitious agenda.

“The American people need IMMEDIATE relief from all of the destructive policies of the last Administration. Speaker Mike Johnson is a good, hard working, religious man,” Mr. Trump wrote in a discursive post that praised himself and his campaign, attacked Democrats and mocked the Rev. Al Sharpton, with whom he has a long and contentious history.

Mr. Trump said Mr. Johnson would “do the right thing, and we will continue to WIN. Mike has my Complete & Total Endorsement. MAGA!!!”

A person close to Mr. Trump, speaking on the condition of anonymity, said the fact that House Republicans had defied the president-elect on the debt limit demand helped Mr. Trump realize that some members of the House posed challenges, but that Mr. Johnson was not one of them.

Mr. Trump and Mr. Johnson have been talking by phone regularly, and the speaker has told associates that the conversations have been warm and supportive.

Before making his endorsement on Monday, Mr. Trump had privately told people that Mr. Johnson had asked for his support, but that he was not sure he was going to back him. Mr. Trump and his advisers also told associates, though, that they did not see who else could get the 218 votes required to become speaker. No Republican has emerged to challenge him so far, though several have publicly vented their dissatisfaction with his performance and complained that Mr. Johnson did not keep them informed about his spending proposal.

After Mr. Trump’s announcement, Representative Jim Jordan, the Ohio Republican who would be a potential alternative candidate, said on social media that he agreed with Mr. Trump’s endorsement as a way to move quickly on the Republican agenda.

“Time to do what we said we would do.” Mr. Jordan wrote on social media.

But Representative Victoria Spartz, Republican of Indiana, said on Fox News on Monday before Mr. Trump’s endorsement that she was uncommitted on supporting Mr. Johnson for the speakership. She reiterated that view later in a social media post.

“I understand why President Trump is endorsing Speaker Johnson as he did Speaker Ryan, which is definitely important,” Ms. Spartz wrote. “However, we still need to get assurances that @SpeakerJohnson won’t sell us out to the swamp.”

After winning the House majority in 2022, Republicans got off to a rough start in 2023, taking 15 ballots and four days to elect Mr. McCarthy as speaker, only to see him deposed 10 months later. That led to Mr. Johnson’s surprise election after others on Mr. McCarthy’s leadership team were rejected on the House floor. A similar scenario in the new Congress would be an embarrassing spectacle for Republicans who will control the White House and both chambers of Congress.

In private, Mr. Trump has fumed about the fact that Mr. Johnson failed during the end-of-session negotiations this month to deliver on his late demand that a year-end spending package also contain a suspension of the debt ceiling, which would have spared him from having to address with the federal borrowing limit when he takes office.

Mr. Trump had made that demand after he and his ally, Elon Musk torpedoed the original catchall spending deal Mr. Johnson had cut with Democrats. Mr. Musk also endorsed Mr. Johnson keeping his job on Monday.

Mr. Trump has complained about a broad fiscal deal negotiated by Mr. McCarthy and President Biden in May 2023, which increased the debt ceiling for two years while freezing and capping some government spending. He has also complained that Mr. Johnson has failed to find a way to spare him a debt ceiling dilemma early in his next term, given that many Republicans refuse to support raising the limit no matter who is in the White House.

The Republican resistance to suspending the debt limit showed that despite Mr. Trump’s demands, dozens of the most conservative House Republicans were willing to oppose him and risk the backlash when it comes to a central issue for them. Mr. Massie was among those who balked at the debt limit request, and he renewed his opposition to Mr. Johnson after the president-elect’s endorsement.

“I respect and support President Trump, but his endorsement of Mike Johnson is going to work out about as well as his endorsement of Speaker Paul Ryan,” Mr. Massie wrote on social media, referring to the former speaker who ended up at odds with Mr. Trump. “We’ve seen Johnson partner with the democrats to send money to Ukraine, authorize spying on Americans, and blow the budget.”

Mr. Johnson did rely on Democratic votes multiple times to push through spending bills to avoid government shutdowns and keep aid flowing to Ukraine. The latter position won him Democratic backing when Mr. Massie sought unsuccessfully to topple Mr. Johnson in May. But Democrats, who were outraged that Mr. Johnson abandoned the spending deal he had struck with them this month, have made it clear they would not rescue him a second time.

Mr. Johnson said he was “honored and humbled” by the incoming president’s backing.

“Together, we will quickly deliver on your America First agenda and usher in the new golden age of America,” Mr. Johnson wrote on social media as he reposted Mr. Trump’s endorsement. “The American people demand and deserve that we waste no time. Let’s get to work!”