Republican Matt Gaetz has filed a resolution to oust Kevin McCarthy as speaker of the US House, setting up a likely showdown vote in the days ahead.
The far-right representative from Florida has threatened for months to use a procedural tool, called a motion to vacate, in a bid to strip his fellow party member of office.
Those threats escalated over the weekend after Mr McCarthy relied on Democrat support for the votes needed to agree a bill to fund the government and avoid a federal shutdown.
Mr Gaetz followed through on those threats by filing a resolution on Monday.
In an earlier speech on the House floor, Mr Gaetz demanded the Speaker disclose the details of a supposed deal he had made with the White House to bring forward legislation to help fund the war in Ukraine during funding negotiations.
He said: “It is becoming increasingly clear who the speaker of the House already works for and it’s not the Republican Conference.”
Brushing off the threat, Mr McCarthy told reporters earlier at the Capitol: “I’m focused on doing the work that has to be done.”
He added that there was “no side deal” on Ukraine, noting he has not spoken to President Joe Biden.
A motion to vacate is a rare and strong procedural tool which has only been used twice in the past century against Republican speakers.
But in recent years, conservatives have wielded the motion as a potential weapon against their leaders.
Mr McCarthy, hoping to appease some on the hard right like Mr Gaetz as he fought to gain their vote for speaker, agreed in January to give as few as five Republican members the ability to initiate a vote to remove him.
When that was not good enough for his critics, he agreed to reduce that threshold to one — the system that historically has been the norm.
That decision has set Mr McCarthy up for the ultimate test of his leadership as he will now have to rely on Democrats to withhold their support for any effort to force his removal.
Because the motion is a privileged resolution, it has priority over other measures and the next step for House leaders is to schedule a vote within two legislative days.
If procedural motions do not slow or stop the process, it would take a simple majority of the House to remove Mr McCarthy from his post.
