They were deeply involved in President Donald J. Trump’s efforts to overturn the 2020 presidential election results. They have come to the defense of people being prosecuted for participating in the deadly storming of the Capitol on Jan. 6, 2021. Some have called for violence against their political enemies online, embraced conspiracy theories or associated with white supremacists.

Several of the most extreme Republicans in Congress and those most closely allied with Mr. Trump have landed seats on the Oversight and Accountability Committee, the main investigative organ in the House. From that perch, they are poised to shape inquiries into the Biden administration and to serve as agents of Mr. Trump in litigating his grievances as he plots his re-election campaign.

Their appointments are the latest evidence that the new Republican majority is driven by a hard-right faction that has modeled itself in Mr. Trump’s image, shares his penchant for dealing in incendiary statements and misinformation, and is bent on using its newfound power to exact revenge on Democrats and President Biden.

Many of the panel’s new Republican members — including Representatives Marjorie Taylor Greene of Georgia, Paul Gosar of Arizona, Lauren Boebert of Colorado and Scott Perry of Pennsylvania — are among Mr. Trump’s most devoted allies in Congress. Their appointments underscore that, while the former president may be a shrunken presence in the current political landscape, he still exerts much control over the base of his party.

They are also an unmistakable signal from Speaker Kevin McCarthy, who won his post after an excruciating battle with hard-right rebels, that he plans to reward such lawmakers — even some who led the opposition to his election — with high-profile roles.

Mr. McCarthy, who credited Mr. Trump with getting him over the finish line in the speaker’s race, said last week that he would study the idea of expunging the former president’s impeachment record.

“I would understand why members would want to bring that forward,” Mr. McCarthy said at a news conference last week, while outlining other priorities of the new Republican majority. “We’d look at it.”

Racists Galore:

Both Ms. Greene and Mr. Gosar were removed from congressional committees by Democrats during the last Congress for internet posts that advocated violence against their political enemies. Both also have appeared with Nick Fuentes, the white supremacist and Holocaust denier.

Ms. Greene, who is pressing to impeach Mr. Biden and has demanded an investigation of the treatment of Jan. 6 defendants, had listed the Oversight panel as her first choice. She recently said that if she had led the Jan. 6 attack, “we would have won” and that people would have been “armed.” (She later said she was being sarcastic.) Mr. Gosar has referred to members of the mob that stormed the Capitol as “peaceful patriots.”

Joining them on the panel will be Mr. Perry, who was one of the key figures in Mr. Trump’s effort to subvert the election results, and Ms. Boebert, who has repeated Mr. Trump’s false claims that the election was stolen and came under fire for posting about Speaker Nancy Pelosi’s location on Twitter during the Capitol riot.

The Oversight Committee has long been populated by the most ideological and outspoken members of the House in both political parties, along with those who have less interest in legislating than in landing political blows that will grab the attention of the public and tarnish their opponents.

“We always treated it as a dumping ground for our less serious members,” said Brendan Buck, who served as a top adviser to the past two Republican speakers, Paul Ryan of Wisconsin and John A. Boehner of Ohio. “Republicans have long treated Oversight as the land of misfit toys.”

But with Democrats controlling the White House and Senate, leaving the G.O.P. with little prospect of enacting its right-wing legislative agenda, House Republicans have made it clear that investigations will be their primary focus, giving members of the Oversight panel more relevance.

“There’s very little evidence that members on the far right have moved on from Donald Trump,” said Mr. Buck. “This will be a forum for his grievances and going down ridiculous rabbit holes and entertaining conspiracy theories.”

But the implications go beyond the committee itself and reflect the state of the party, where moderate voices are few and being on the right side of Mr. Trump is still regarded as a necessity. In 2021, eight Republican senators and 139 Republican representatives voted to sustain one or both objections to the election results that made Mr. Biden president.

“It’s a snapshot of where the Republican Party is,” said William Kristol, a prominent Never Trump conservative, referring to the makeup of the House Oversight Committee. “It’s wishful thinking to think there is a healthy Republican Party and this wacky Republican conference. They just got elected. Aren’t they the most representative thing of the party that exists?”

Mr. McCarthy said two years ago that Mr. Gosar and Ms. Greene would have their committee assignments restored, and possibly elevated, if Republicans won back control of the House. “They may have better committee assignments,” Mr. McCarthy said at the time.

The speaker also credits himself with putting people in positions where they can be most successful. He may see a political upside to placing politically motivated firebrands on a committee devoted to investigations that could tarnish Mr. Biden. In 2015, Mr. McCarthy jeopardized his first attempt at becoming speaker by bragging that the House select committee formed to investigate the attack on the American diplomatic mission in Benghazi, Libya, had been successful in damaging Hillary Clinton, the Democratic presidential candidate.

The White House has seized on the elevation of members who have mimicked Mr. Trump’s tactics as the latest example of the Republican Party’s lurch to the extremes.

“Republicans are handing the keys of oversight to the most extreme MAGA members of the Republican caucus who promote violent rhetoric and dangerous conspiracy theories,” Ian Sams, a White House spokesman, said in a statement, referring to Mr. Perry, Ms. Greene, Ms. Boebert and Mr. Gosar. “They have defended and downplayed a violent insurrection against our democracy.”

Mr. Sams added, “House Republican leaders should explain why they are allowing these individuals to serve on this committee and reveal transparently once and for all what secret deals they made to the extreme MAGA members in order to elect a speaker.”

Other new members on the Oversight Committee who have attracted less attention include Representative Russell Fry of South Carolina, who has campaigned with election conspiracy theorists including Mike Lindell, the chief executive of MyPillow; and Representative Anna Paulina Luna of Florida, who has denied the results of the 2020 election and has appeared on a television program that has pushed the QAnon conspiracy theory.

A spokeswoman for Ms. Luna said the congresswoman “has done thousands of hours of media on the campaign trail and as a member of Congress and, being that she works full time, does not obsessively track TV programs.”

It is not yet clear how much latitude lawmakers devoted to Mr. Trump will have to use the panel to do his bidding. Some Republicans have signaled, for instance, that they do not want to relitigate the work of the Jan. 6 committee, as Mr. Trump has made it clear he desperately wants to do, fearing that focusing on the attack and the former president’s actions leading up to it will hurt the party politically.

Representative James R. Comer, Republican of Kentucky and chairman of the Oversight Committee, has said he plans to investigate Mr. Biden’s family and its business connections. He is not seen as an extremist in the conference.

But in his role as chairman, he will have to balance and address the demands of committee members like Ms. Greene, who has already introduced five articles of impeachment against Mr. Biden. That includes one on the day he took office, when she accused him of abusing his power as vice president to benefit his son Hunter Biden’s business dealings in Ukraine.

Mr. Comer “may have claimed that he wanted the committee to be ‘credible,’ but the selection of these members shows this committee is nothing more than a bad joke,” said Brad Woodhouse, a senior adviser to the Congressional Integrity Project, a group dedicated to undermining Republican-led congressional investigations.

In a statement on Wednesday evening, Mr. Comer called Republicans on his panel “an all-star lineup ready to hit the ground running and go to bat for the American people.”