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 that 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 been vowing 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 brokered by McCarthy over the weekend running out by mid-November.