The US Capitol's police chief and the attorney general reassured Americans on Wednesday that they have addressed the security collapse behind the January 6 attack on Congress -- even if the anger and conspiracy theories fueling the insurrection are stronger than ever.

On the eve of the first anniversary, the man in charge of security at the Capitol, Thomas Manger, said his forces would not be caught unprepared again, as they were when thousands of Donald Trump's supporters stormed Congress to try and stop certification of Joe Biden's presidential victory.

"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."

Later, Attorney General Merrick Garland will spell out efforts to "hold accountable those responsible" for the mayhem and underline law enforcement's "unwavering commitment to defend Americans and American democracy," the Justice Department said.

However, despite hundreds of criminal probes into the unrest, reform of the Capitol police and stockpiling of riot gear in case of any repeat attack, Biden will give the keynote speech of the anniversary on Thursday knowing that much of the country still celebrates the insurrection.

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

Senate Republican leader Mitch McConnell was set to be away in Atlanta, Georgia, attending the funeral of the late senator Johnny Isakson, his office said.

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."

Biden and Vice President Kamala Harris will speak from inside the Capitol, the setting during the unrest of almost unbelievable scenes as Trump supporters fought past police to invade the heart of US democracy last year.

"He'll speak to the historical significance of January 6, what it means for the country one year later," White House Press Secretary Jen Psaki said.

'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, then roamed the halls, 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."

Police "displayed resilience, fortitude, and unimaginable bravery. Together with our federal, state, and local law enforcement partners, they fought under conditions that, in my four decades of law enforcement, I have never witnessed," Manger said.

"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," he 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."

'Unprecedented'

While Trump is retreating from the public eye on the anniversary itself, he said he would return to the issue at a rally planned in Arizona on January 15.

Despite losing by more than seven million votes to Biden, Trump continues to insist that he was the real winner in 2020.

The claim is only the most incendiary element of a broader attack against Biden on everything from immigration to Covid-19, all adding up to what looks very much like an as-yet undeclared bid to take back power in 2024.

It's a campaign that Carl Tobias, a professor at the University of Richmond School of Law, calls "unprecedented in US history."

"No former president has attempted to do so much to discredit his successor and the democratic process," Tobias said.