Democrats marked the anniversary of the US Capitol riot on Friday by linking the violence to the internecine warfare that has broken out among Republicans paralysing the House of Representatives over the selection of the next speaker.

The Republicans, who have a razor-thin majority in the lower chamber of Congress, were entering a fourth day of infighting over establishment favourite Kevin McCarthy, who has lost a historic 11 ballots for the job, with some 20 of the party's hardline conservatives blocking his path.

Senate Majority Leader Chuck Schumer said the second anniversary of the insurrection – when a mob of defeated president Donald Trump's supporters ransacked the Capitol – should serve as a "wakeup call" for Republicans to reject extremism.

But he added: "Unfortunately, the utter pandemonium wrought by House Republicans this week is just one more example of how the extreme fringe of their party, led by election deniers, is pulling them further into chaos and making it impossible for them to govern."

The top Democrat spoke out as President Joe Biden was preparing to award medals at a sombre ceremony acknowledging police officers who defended the Capitol.

Former Washington police officer Michael Fanone, who has become a vocal critic of Trump and his allies in Congress after being assaulted by the insurrectionists, said the crisis in the House had demonstrated that Republicans were likely to be "fractious, even ungovernable."

"If Republicans can finally agree on a speaker, the same... leaders who spread former president Donald Trump's lies about the 2020 election – and who have consistently downplayed the grave threat to the nation posed by the January 6, 2021 insurrection carried out in his name – will take the reins of power in the House," Fanone wrote in an op-ed for CNN.

As House Democrats gathered on the Capitol steps to hear a message from their leader Hakeem Jeffries on the insurrection, McCarthy was facing down what was likely to be another failed bid to be elected the country's most senior lawmaker.

There have already been more rounds of voting than any speaker election since the Civil War.

Weary lawmakers-elect on all sides were hoping the fight would soon be approaching a tipping point, when a significant bloc of conservatives move in McCarthy's direction – or his quest to become speaker withers on the vine.

McCarthy says he isn't expecting to adjourn the House until the speaker's race is wrapped up, threatening a working weekend for his frustrated troops.

"We're going to make progress. We're going to shock you," the 57-year-old Californian told reporters as he walked into the Capitol on Friday. (AFP)