A far-right lawmaker launched a bid on Monday to oust US House Speaker Kevin McCarthy from his post, triggering the most serious showdown yet within the Republican party as it battles to contain firebrand supporters of ex-president Donald Trump.

Matt Gaetz, a representative from Florida, entered his resolution from the House floor, later telling reporters McCarthy "doesn't have my support anymore, and he doesn't have the support of a requisite number of Republicans to continue as the Republican Speaker."

"Bring it on," McCarthy quickly responded on X, formerly known as Twitter.

The resolution, a rare maneuver which requires a simple majority to pass, could force a vote within days and comes after McCarthy brokered a last-gasp budget deal with Democrats over the weekend, temporarily avoiding a government shutdown.

Gaetz has vowed retribution since the deal was struck.

The Florida congressman and Trump loyalist is a leading figure within the small group of far-right Republican legislators who ignited the shutdown crisis with their refusal to adopt fresh federal funding without deep spending cuts.

The group also opposes sending additional support to Ukraine, saying the money would be better spent in the United States on combatting illegal immigration.

Republicans have such a narrow majority in the House of Representatives, the lower house of Congress, that the hardliners -- though they are a small minority -- hold a de facto veto over legislation there.

"The American people are tired of Washington DC not having a budget, running $2 trillion annual deficits sitting atop a $33 trillion debt," Gaetz told reporters Monday after introducing his motion.

"If this country's going down, I'm going down fighting," he said.

- 'Stop playing games' -

The resolution throws up fresh obstacles as lawmakers race to cut a new deal on funding for the federal government, with the stopgap agreement brokered by McCarthy over the weekend running out by mid-November.

The last-ditch "continuing resolution" came as millions of public workers looked set to be sent home unpaid, upending government functions from military operations to food aid to federal policymaking.

US President Joe Biden has lambasted both McCarthy and the hardliners for failing to live up to an agreement forged around a debt crisis months ago that was meant to avoid a damaging shutdown fight -- and for stripping out support for Ukraine.

"Stop playing games, get this done," Biden said during an address on Sunday, adding that he was "sick and tired of the brinksmanship, and so are the American people."

The timeline for next steps on Gaetz's resolution is variable, and it is not yet clear if Democrats would come to McCarthy's aid.

Gaetz allowed for the possibility as he talked with reporters Monday, but vowed McCarthy "is not going to be speaker in power as a consequence of Republican votes."

He added that "if the Democrats want to own Kevin McCarthy, they can have him. Because one thing I'm at peace with is -- when we stand here a week from now, I won't own Kevin McCarthy anymore."

Gaetz also called for the border funding and Ukraine aid to be voted on separately.

"Regardless of how you feel about Ukraine money or border money, these two things should not be lumped together. They deserve their own dignity and their own vote," he said.

McCarthy made concessions to the bloc in January in return for their support in his quest to gain the speaker's gavel -- among them, a rule change that allows any member to file a such a "motion to vacate."

"This has nothing to do with a personality," Gaetz said Monday, when asked if he was motivated by a vendetta against McCarthy.

"This has to do with breach of an agreement. I laid that breach out weeks ago."