President Joe Biden will turn the screws on Donald Trump by calling him out Thursday for "singular responsibility" in provoking the mayhem on January 6 last year, when the outgoing Republican leader's supporters stormed Congress.

Biden's decision to use his speech on the anniversary to squarely blame Trump and Republican allies for their role in the unprecedented attack on US democracy will mark a sharp escalation in Biden's approach to Trump and the riot.

Through the first year of his presidency, Biden has referred mostly to ignore Trump, who still refuses to acknowledge his defeat in the 2020 presidential election and continues to spread conspiracy theories to his millions of followers about being the true winner.

But in a speech from inside the Capitol's Statuary Hall -- where exactly a year ago a Trump mob rampaged through to try and stop certification of Biden's election win -- the Democratic president will firmly blame his predecessor, Press Secretary Jen Psaki said Wednesday.

"I would expect President Biden to lay out the significance of what happened at the Capitol and the singular responsibility President Trump has for the chaos and carnage," she said.

"President Biden has been clear-eyed about the threat the former president represents to our democracy," she said.

Asked if Biden would use Trump's name, Psaki said "we're finalizing the speech, but I think people will know whom he's referring to."

Earlier Wednesday, the Capitol police chief, Thomas Manger, said his forces would not be caught unprepared again, as they were last year.

"I am confident that the US Capitol Police is a stronger, better prepared law enforcement agency," Manger told a Senate hearing, recalling the desperate struggle his officers put up against "a violent mob and vastly outnumbered."

And Attorney General Merrick Garland also promised that justice was being done, saying he was "committed to holding all January 6th perpetrators, at any level, accountable under law -- whether they were present that day or were otherwise criminally responsible for the assault on our democracy."

Senior Republican skips anniversary

However, in the political sphere, the country remains dangerously divided.

Underlining the stunning partisan split, senior Republicans appear set to skip commemorations organized by the Democratic-led Congress on Thursday, including Biden's speech and a prayer vigil.

Senate Republican leader Mitch McConnell and others from his party were set to be away in Atlanta, Georgia, attending the funeral of the late senator Johnny Isakson.

In an opinion piece on Fox News, Republican Senator Josh Hawley, a leading supporter of false claims that Biden's 2020 victory was suspect, referred to January 6 as a "demonstration" that "some demonstrators" marred.

"Gathering for a political demonstration is not a crime. On the contrary, it is a right expressly protected by the US Constitution," he wrote.

Trump himself continues to push the lie that the 2020 election was stolen by Biden -- a conspiracy theory dismantled in multiple court findings and vote recounts.

He had been planning a press conference at his Florida home to steal the limelight on Thursday, but abruptly abandoned the plan in a statement Tuesday that nevertheless continued to claim that the 2020 election was a "crime."

"Hand to hand"

In his testimony, Manger paid tribute to his badly prepared officers as they tried to face down angry Trump supporters, who poured over police barricades, smashed windows to get into the Capitol, vandalizing offices and the chamber itself.

For hours as the struggle unfolded, lawmakers fled to safety or hid behind barricaded doors. Trump's vice president Mike Pence, who was in the Capitol to preside over the certification of Biden's win, fled as attackers chanted "hang Mike Pence."

"Their eyes inflamed by repeated shots of pepper and bear spray, their bodies assaulted and beaten with bricks, flag poles, rebar, pipes, bats, sticks, Tasers, among other weapons, they fought for over four hours," Manger said.

"Fighting hand-to-hand, using ingenuity and displaying incredible grit, they did not give up. Congress was able to do its job and not one Member, or staff, was physically harmed."

Democratic Senate Majority Leader Chuck Schumer recalled on Wednesday how a police officer had grabbed him during the unrest and told him they were "in danger."

"I was within 30 feet of these nasty insurrectionists," Schumer said, describing the attack as an attempt to reverse "the outcome of a free and fair election."

"Without addressing the root causes of the violence on January 6, the insurrection will not be an aberration it could well become the norm," he warned.