Republican Senate leader Mitch McConnell denounced fellow GOP member and newly elected Representative Marjorie Taylor Greene on Monday, calling the far-right Georgian's embrace of conspiracy theories and "loony lies" a "cancer for the Republican Party".
"Somebody who's suggested that perhaps no airplane hit the Pentagon on 9/11, that horrifying school shootings were pre-staged, and that the Clintons crashed JFK Jr's airplane is not living in reality," said Mr McConnell.
"This has nothing to do with the challenges facing American families or the robust debates on substance that can strengthen our party."
The statement comes as House Democrats are mounting an effort to formally rebuke Ms Greene, who has a history of making racist remarks, embracing conspiracy theories and endorsing violence directed at Democrats. It also puts pressure on House Republican leaders to discipline her.
Democrats have teed up action on Wednesday to send a resolution to the House floor that would strip Ms Greene of assignments on the House education and budget committees, if Republican House Minority Leader Kevin McCarthy does not do so first.
"It is my hope and expectation that Republicans will do the right thing and hold Representative Greene accountable, and we will not need to consider this resolution," said Democratic House Majority Leader Steny Hoyer. "But we are prepared to do so if necessary."
Some Democrats have called for going further and expelling Ms Greene from the House — an unlikely outcome that would require backing from Republicans, since expulsion requires a two-thirds vote. Another option is censure.
Democrats' willingness to act against a member of the opposing party underscores their desire to confront far-right politicians, like Ms Greene, who are closely aligned with some of former President Donald Trump's fringe supporters, including extremist groups that were involved in the violent Capitol insurrection.
It also shines a light on the GOP's reluctance to punish Trump supporters in their ranks for fear of alienating some of the former president's most ardent voters.
"If Republicans won't police their own, the House must step in," said Democratic Representative Debbie Wasserman Schultz, who is sponsoring the measure to remove Ms Greene from the committees.
In a tweet over the weekend, Ms Greene sounded a defiant tone. She also said she had spoken to Mr Trump and was "grateful for his support".
"I will never back down and will stand up against the never ending blood thirsty mob," she tweeted.
On Monday, she tweeted that Democrats, if they move forward, would come to regret the "precedent they are setting", arguing it would be "used extensively against members on their side once we regain the majority after the 2022 elections".
Ms Greene's views were in the spotlight even before she joined the House last month.
The real cancer for the Republican Party is weak Republicans who only know how to lose gracefully.
This is why we are losing our country.
— Marjorie Taylor Greene 🇺🇸 (@mtgreenee) February 2, 2021
The Georgia Republican has expressed support for QAnon conspiracy theories, which focus on the debunked belief that top Democrats are involved in child sex trafficking, Satan worship and cannibalism.
Facebook videos surfaced last year showing she had expressed racist, anti-Semitic and anti-Muslim views. Top Republicans denounced her at the time, hoping to block her from capturing the GOP nomination in her reliably red congressional district in north-west Georgia.
But after she won her primary, they largely accepted her. Since then, even more of her past comments, postings and videos have been unearthed, though many were deleted recently after drawing attention.