Six months after a mob of Donald Trump's supporters stormed the seat of American democracy in the worst attack on the legislature since the War of 1812, the American public will learn of the challenges faced by law enforcement in the debut hearing before a select committee that has become a political flashpoint.

"Nothing will be off-limits. We will do what is necessary to understand what happened, why, and how," the panel's Democratic chairman, Bennie Thompson, wrote in an opinion piece in the Washington Post.

"The committee will provide the definitive accounting of one of the darkest days in our history."

Lawmakers will receive first-hand accounts from police attacked by rioters who fought their way into the building, hunted for the likes of House Speaker Nancy Pelosi, and sought to block certification of Joe Biden's November presidential election victory.

Trump on Monday dismissed the probe as "a fake and highly partisan" and attempted to blame Pelosi for allegedly failing to protect the Capitol from his supporters.

Four police officers will testify at the 9:30 am (1330 GMT) hearing, including Washington officer Michael Fanone, who was stun-gunned and beaten by rioters.

Fanone, who suffered a heart attack during the mayhem, has told US media that the clashes amounted to "the most brutal, savage hand-to-hand combat" of his life.

Also testifying is US Capitol Police officer Harry Dunn, who has spoken of racial epithets hurled at him and other police by rioters, many of whom were associated with ultra-nationalist and white supremacist groups.

Five people died during or shortly after the insurrection, while dozens of police were injured.

House Republican leadership has essentially boycotted the select committee, pulling its five appointments last week after Pelosi took unprecedented action by rejecting two of Minority Leader Kevin McCarthy's picks.

Instead of leaving the panel with just Democrats, Pelosi unilaterally named two Republicans: Liz Cheney and Adam Kinzinger.

Both are forceful Trump critics who voted for his impeachment in January, and both have drawn GOP censure for refusing to back Trump's baseless claims that the election was stolen.

Pelosi and others had wanted a bipartisan, independent 9/11 commission-style panel to investigate the riot and its origins. Even McCarthy in January voiced support.

But with anxiety growing among Republicans concerned that a January 6 probe could prove politically damaging for their party ahead of the 2022 midterm elections, the party began coalescing against a deep dive.

Senate Republicans in May blocked the commission, arguing that multiple investigations have already reached conclusions about the riot and hundreds of arrests have yielded considerable data about what happened.

The party has sought to destroy the credibility of Pelosi's subsequent committee, with McCarthy accusing her of "playing politics" as partisan tensions soar.

"Never in the history of America has a speaker picked the other side, so they are predetermining what comes out" from the probe, McCarthy told reporters Monday.

He also derided conservatives Cheney and Kinzinger as "Pelosi Republicans," a slight they dismissed as "childish."

'Whitewash history'

The speaker insists the committee will forge ahead -- with or without more Republican engagement.

"We have to, again, ignore the antics of those who do not want to find the truth," she told ABC Sunday.

Democrats like committee member Adam Schiff said they are eager to learn about the actions of Trump and some Republican lawmakers before and during the riot.

Schiff said he expects the panel to use subpoena power to compel appearances of witnesses reluctant to testify.

"It's all the more important now that people are trying to whitewash history," Schiff told CNN.

Biden meanwhile is backing Pelosi's strategy, "which is to get to the bottom of what happened and prevent it from happening in the future," White House Press Secretary Jen Psaki said.