Republican frontrunner Kevin McCarthy suffered a shock defeat in the race to become the new speaker of the US House of Representatives late Friday amid astonishing scenes of bitter infighting after failing to stamp out a rebellion among his party's ranks.

McCarthy had been expected finally to win a majority to lead the Republican-controlled House after four days of coming up short but his victory lap was derailed as he failed by just one vote out of more than 400 cast.

As Matt Gaetz voted "present" to deny McCarthy the gavel, the disappointed Republican leader went over to talk to the Florida lawmaker-elect.

Gaetz pointed a finger at McCarthy, who began retreating as Alabama's Mike Rogers lunged at Gaetz and had to be held back.

Amid the chaos, a vote to adjourn failed and the House was expected to move to a 15th voting round -- which McCarthy could win.

"Take a seat, let's go one more time," McCarthy was heard saying, as his members chanted back: "One more time."

The Republicans, who hold a razor-thin majority, had been mired in internecine warfare as McCarthy lost consecutive ballots for the prestigious role, with around 20 conservative hardliners blocking his path since Tuesday.

But the 57-year-old Californian was able to pick up more than a dozen votes among the defectors in the 12th and 13th rounds on Friday after offering major concessions.

McCarthy claimed that he had the support to finally secure the gavel -- only to be humiliated in a floor vote given wall to wall coverage on US news channels.

"Just reminds me of what my father always told me," McCarthy had told reporters. "It's not how you start, it's how you finish. And now we have to finish for the American public."

- Confidence -

There have been more rounds of voting in the fractious, bitter 2023 contest than in any speaker election since the Civil War.

McCarthy has projected confidence all week, even as he was bleeding votes rather than adding to the base of around 200 Republicans who have backed him all along.

His party's takeover of Congress is expected to herald the end of cross-party cooperation, with the legislative process gridlocked and Republicans promising an aggressive agenda of investigations into most aspects of President Joe Biden's administration and his family.

Democrats and some of McCarthy's own supporters, in private, are concerned that he has been offering his far-right critics radical policy commitments that will make the House ungovernable.

There were reports, which AFP has not verified, that he had agreed to propose keeping spending at 2022 levels, including a cap on military funding which would have the same effect as a $75 billion cut.

That has raised alarm among defense hawks pushing for the United States to project strength amid Russia's invasion of Ukraine and an emboldened Chinese stance on Taiwan.

No single lawmaker, however senior, has the authority to set budgets, but the fact that the suggestion was being taken seriously underscores the Republicans' turn towards isolationism under the leadership of Donald Trump.

Other lawmakers-elect were complaining that McCarthy was handing the hardliners plum committee posts and changes to the rules that would severely curtail the role of the speaker.

- Poisoned chalice -

The renegade Republicans are understood to have flipped their votes in exchange for rule changes making it possible to oust the speaker in a vote called by just one member.

They are also asking for an outsized role in deciding which bills make it to the floor and how they are handled.

The length and precariousness of the speaker selection process has highlighted how difficult McCarthy is going to find it to corral votes in the 118th Congress should he eventually be elected.

Democrats said the role would be a poisoned chalice, as the compromises McCarthy has made would leave him as the weakest speaker in modern history.

"He has moved steadily to the right and he has capitulated at every turn to these extremist elements in the GOP," Maryland congressman Jamie Raskin told MSNBC, referring to the Republicans by their nickname, the "Grand Old Party."