Republican favorite Kevin McCarthy failed in his ninth attempt to win speakership of the U.S. House of Representatives in a three-day stand-off that has led to a political crisis. The House moved to hold the 10th round, the most since 1859.

McCarthy, a favorite of the Republican establishment but a bete noire of the far right, made sweeping concessions overnight to quell a rebellion of around 20 hardliners in his own camp blocking his bid to be the country's top lawmaker.

But his overtures appeared to have fallen on deaf ears as he failed to win over a single opponent in Thursday's opening vote, the eighth since the chamber opened for a new term under a thin Republican majority.

The 57-year-old Californian had already been humiliated by failure to secure the gavel across six voting rounds during a chaotic, rollercoaster 48 hours, falling behind each time to his Democratic counterpart – Hakeem Jeffries – though neither was able to win the required majority.

"It's my hope that today the House Republicans will stop the bickering, backbiting and backstabbing so we can have the backs of the American people," Jeffries told reporters at the U.S. Capitol.

McCarthy crossed one of his red lines overnight by agreeing to lower the threshold needed to force a vote ousting a speaker from a majority of either party to just one member, thereby imperiling his chances of a long tenure.

He also reportedly signaled that he is willing to give the ultra-conservative House Freedom Caucus two or three seats on the powerful Rules Committee, the speaker's mechanism for controlling how floor votes are conducted.

His opponents also claim to have extracted a pledge to limit lawmakers to three terms, and for a McCarthy-linked political organization to stop putting up moderates against far-right candidates in safe Republican seats.

US security 'at risk'

The 2023 speakership race is the first in a century to require multiple rounds of voting. No House business can take place without its presiding officer in place, meaning lawmakers-elect have to continue voting until someone wins a majority.

Until then, the chamber will be unable to swear in members, set up committees, tackle legislation or open any of the investigations Republicans have promised into President Joe Biden.

Three Republican lawmakers due to head national security committees also warned in an open letter on Thursday that the House is currently unable to conduct oversight of the Pentagon or intelligence community.

"We cannot let personal politics place the safety and security of the United States at risk," they said.

McCarthy has long dreamed of being speaker, but Tuesday and Wednesday were among the most humiliating days in his career as he failed to win a majority in vote after vote, despite Republicans controlling the House.

In the only significant change in the voting, Donald Trump, who is bidding to return to the White House and does not harbor speakership ambitions, secured his first nod, from Florida congressman Matt Gaetz.

McCarthy's failure is being seen as a further marker of Trump's weakening hold over the party, as the lawmaker's vote share actually dropped after he was endorsed by the former president on Wednesday.

Texas conservative Chip Roy had indicated that McCarthy's proposed compromises could reduce the so-called "Taliban 20" opposing his speakership bid to around 10 detractors.

'Dug in'

But in the end 21 of his 221 Republican colleagues opposed him – the same number as in all three of Wednesday's votes – and he can only afford to lose four.

In many cases, McCarthy's critics lack specific objections to his policies, but claim instead that they find him untrustworthy, lacking political philosophy and motivated only by a desire for power.

The House can move straight to an eighth vote for the speakership, although the McCarthyites may try to seek an adjournment to continue talks.

The top Republican's allies are still hoping that a reduction in opposition to single figures in upcoming voting rounds might increase pressure on the remaining holdouts to follow suit. But others fear the risky strategy of giving away the store to the most extreme fringe of the party will eventually spark a backlash among moderates.

McCarthy-friendly Texan Pete Sessions told CNN on Wednesday his side could only afford three or four more inconclusive votes before they needed to start looking for a less divisive candidate. "I'm telling you these 19 people are dug in," he said of the rebel group of "Never Kevins."