US President Joe Biden and first lady Dr. Jill Biden participate in an event to call NORAD and track the path of Santa Claus on Christmas Eve in the South Court Auditorium of the Eisenhower Executive Building on December 24, 2021 in Washington, DC.Photo:AFP

A divided nation will experience an ominous split-screen moment Thursday when President Joe Biden uses the anniversary of the January 6 attack on Congress to warn of threats to US democracy and Donald Trump goes live with his conspiracy theories.

One year after a mob of Trump supporters marched on Congress to try and prevent lawmakers from certifying Biden's victory in the presidential election, political wounds remain far from healed.

Biden and Vice President Kamala Harris will reportedly 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.

As a veteran politician who came out of retirement to take on what he saw as Trump's authoritarian presidency, Biden has often warned during his first year in the White House of an "existential" threat to political freedoms that until now most Americans took for granted.

His speech - part of a series of events on what Biden's key ally, Democratic Speaker of the House Nancy Pelosi, says will be a "difficult day" - is set to take that warning to a new level.

But while Congress is holding a prayer vigil for what Biden has called "a dark moment," Trump will be giving a press conference from his luxury property in Mar-a-Lago, Florida.

His message is likewise easy to predict. 

Despite losing by more than 7 million votes to Biden, and despite losing multiple court challenges around the country, Trump continues to tout wild claims that the 2020 election was stolen.

And the accusations are 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.

However ludicrous the election conspiracy theory may be - one federal judge in Pennsylvania ruled Trump's case "strained" and "speculative" - it is seen as truth by millions of Americans.

Polls consistently show that around 70 percent of Republicans think Biden was elected illegitimately.