It was, Nancy Pelosi told reporters on Thursday, one of the darkest days in America’s history – an assault on democracy, Congress and the constitution. “The American people want to know the truth,” she said.

But will the truth of the 6 January insurrection at the US Capitol ever be fully told?

Pelosi, the Democratic speaker of the House of Representatives, finds herself locked in a battle of wills with Kevin McCarthy, the Republican minority leader who, critics say, is determined to change the political conversation to any other subject.

The two Californians have clashed bitterly this week over the makeup of a special committee to investigate the riot, which disrupted the certification of Joe Biden’s election win over Donald Trump. At stake is not only a full accounting of that day – what role Trump played, why security forces fell short – but also the political class’s ability to investigate itself.

“Clearly Washington today is not capable of getting to the truth,” said Larry Jacobs, director of the Center for the Study of Politics and Governance at the University of Minnesota. “You’ve got such poisonous tribal warfare between Republicans and Democrats that every encounter is one of viciousness.

“The die was cast several months ago when McCarthy refused to support a nonpartisan approach. He doesn’t want to get to the truth. The truth is so damning about Trump and McCarthy doesn’t want to be at odds with Trump. That’s the bottom line political calculation here.”

America was stunned on 6 January when a mob of Trump supporters laid siege to the Capitol, penetrating the Senate chamber and calling for vice president Mike Pence to be hanged. Five people died, more than a hundred were injured and members of Congress ran for the lives as a result of the historic national security failure.

The Democratic-controlled House voted to create a commission, split evenly between the parties, modeled on the body that investigated the September 11 2001 terrorist attacks on New York and Washington. But Republicans in the Senate blocked the proposal in May by deploying a procedural rule known as the filibuster.

The House’s next move was to create its own Democratic-majority select committee to investigate the causes of 6 January, how it was organized, who paid for it, who persuaded thousands of Trump supporters to descend on Washington and what happened when they did. It was opposed by all but two Republicans: Liz Cheney of Wyoming and Adam Kinzinger of Illinois, both outspoken critics of Trump.

Who should sit on the panel is now the subject of a bitter power struggle. On Wednesday Pelosi rejected two Republicans picked by McCarthy: Jim Banks of Indiana and Jim Jordan of Ohio. Both are staunch and unabashed Trump allies who deny his role in the attack.

Banks recently travelled with the former president to the US-Mexico border and visited him at his New Jersey golf course. Jordan was one of Trump’s strongest defenders during his two impeachments and last month described the new investigation to “impeachment three”.

An angry McCarthy denounced Pelosi’s move as “an egregious abuse of power” and said Republicans would not take part at all. He claimed that the panel has lost “all legitimacy” because Pelosi would not allow the Republicans to name their own members.

On Thursday the war of words intensified. At a press conference, Pelosi argued that it would be “ridiculous” to let Banks and Jordan serve on the committee. “They had made statements and taken actions that I think would impact the integrity of the committee, the work of the committee,” she said.

“This is deadly serious. It’s about our constitution, our country. It’s about an assault on the Capitol that’s being mischaracterized for some reason at the expense of finding the truth for the American people.”

Pelosi added: “It is my responsibility as speaker of the House to make sure we get to the truth on this, and we will not let their antics stand in the way of that.”

Later McCarthy returned fire once again. “This is a sham committee that’s just politically driven by Speaker Pelosi,” he told reporters.

But analysts note McCarthy’s loyalty to Trump, his potential to be subpoenaed to testify about a phone conversation with the then president on 6 January, and his personal ambition to replace Pelosi in the speaker’s chair after next year’s midterm elections.

Larry Sabato, director of the Center for Politics at the University of Virginia, said: “Look, I accept hypocrisy – it’s a big part of politics – but this is just dripping with hypocrisy. They stood up and said on or shortly after January 6 that it was a terrible thing and that these people must be brought to justice and now I think they’re going to award them presidential medals of freedom!

“It’s just beyond belief but it’s designed for one thing. They know that they’ll never convince independents, much less Democrats, of their ridiculous point of view about January 6. This is designed to shore up Republicans and to please Trump and it’s working.”

Democrats insist that the investigation will go ahead whether the Republicans in question take part or not, as Pelosi has already appointed eight of the 13 members including Cheney, which gives them a bipartisan quorum to proceed, according to committee rules. It will hold its first hearing next week, with at least four rank-and-file police officers who battled rioters testifying about their experiences.

The standoff is symptomatic of raw political divisions in Congress and raises the prospect that the only comprehensive investigation currently being conducted into the attack will be done almost entirely by Democrats. McCarthy will surely seek to portray it as hopelessly partisan and therefore lacking credibility.

But Kurt Bardella, a former Republican congressional aide who switched to the Democrats, disagrees. He said: “If anything, the omission of January 6 sympathisers will ensure that the committee’s work progresses in a fair, impartial manner free from the stunts and the antics and the interruptions and the disruptions that have become commonplace by Republicans like Jim Jordan.

“Now we’ll be able to have proceedings where we hear from witnesses, where we have a real line of questions that aren’t interrupted, where facts will be allowed to be given a transparent airing for the American people to decide how they feel about the information presented by the committee. The investigative process will be able to happen now unobstructed from interference from January 6 sympathizers.”

Bardella remains optimistic that the committee will be able shed new light on the events of one of America’s darkest days. He added: “The reality is that those complicit in creating the environment that enabled January 6 to happen cannot be expected to be impartial investigators. It would be akin to inviting members of al-Qaida to be on the 9/11 commission.”