Washington DC - Just three days before the House of Representatives chooses its next speaker, Republican Mike Johnson is in an increasingly nerve-racking fight to keep his gavel amid mutterings of rebellion on his right flank.

President-elect Donald Trump endorsed Johnson on Monday, but even that may not be enough for the 52-year-old Louisiana conservative to win over the lower chamber of Congress, where the razor-thin Republican majority can afford almost no dissent.

A chaotic 2023-24 session was marked by anger over Johnson's handling of spending negotiations, as fiscal hawks lined up to attack him on grounds that he ceded too much to Democrats and was soft on the deficit.

Johnson sought to project confidence in his job prospects on Monday, vowing to "get to work" on the incoming president's priorities in the new term.

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"Together, we will quickly deliver on your America First agenda and usher in the new golden age of America," he posted on X. "The American people demand and deserve that we waste no time."

Trump's looming presidential inauguration raises the stakes of Friday's vote, since the House can do nothing until its leader is decided – including completing the certification of the 78-year-old Republican's victory, set for Monday.

Assuming every member is present and voting on Friday and that Democrats all back their leader, Hakeem Jeffries, Johnson can afford only one defection and still be reelected.

But he already has one Republican "no" on the scoreboard, from Kentucky's Thomas Massie, while a handful of other conservative hardliners have been publicly open to a change at the top.

Massie criticized Trump's support of Johnson, accusing the speaker of having partnered with Democrats to "send money to Ukraine, authorize spying on Americans, and blow the budget."

Crucially, the increasingly influential Elon Musk replied to the post, saying that Massie "might be right."

Johnson was a virtual unknown to the wider public before becoming speaker but came to Trump's attention when he spearheaded efforts in Congress to overturn the 2020 election.

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Far from his party's first choice, the attorney and religious rights campaigner won the gavel in 2023 largely because he lacked the enemies on his own side that prompted the downfall of other Republicans.

"Speaker Mike Johnson is a good, hard working, religious man," Trump posted on Truth Social on Monday, as he finally tipped his hand following days of silence. "He will do the right thing, and we will continue to WIN. Mike has my Complete & Total Endorsement."

The move risks further defeat for Trump, who was rebuffed by House Republicans earlier this month as he demanded a suspension of the country's borrowing limit, which would essentially have given him a blank check for his time in office.

"I understand what Mike's up against... if a bad case of the flu comes through there, we're in the minority," Tennessee's Tim Burchett told CNN, without committing to supporting Johnson.

"I mean, that's the reality of Washington today, and he has some tough decisions. I think ultimately it's going to be decided (by) who President Trump likes, because I believe that will weigh in heavily on the decision-making."

Johnson is already an old hand at leadership challenges, after Trump ally Marjorie Taylor Greene pushed a "motion to vacate" the speaker's chair last May and he had to be saved by Democrats.

Jeffries has been clear that he won't be coming to the rescue this time, however.

On the other hand, no credible challengers have emerged and there is little appetite for a repeat of the chaos that attended leadership elections in the last Congress.

Republican rebels brought down then-speaker Kevin McCarthy in 2023, leading to three weeks of party infighting that paralyzed Congress, before Johnson was elected.