"Whether democracy is still America's sacred cause is the most urgent question of our time," Mr Biden said.

"It's what the 2024 election is all about," he added.

The speech saw Mr Biden returning to a theme he has invoked over and over in recent years.

This time, he explicitly drew a line to the 6 January 2021 attack on the US Capitol to make his case.

That day, Mr Trump's supporters violently stormed Congress to stop lawmakers from certifying the presidential election results for Mr Biden, just weeks before he was set to take office.

Mr Trump, the current frontrunner for the Republican presidential nomination, frequently repeats the false claim that the 2020 election was stolen from him.

The former president has also attempted to reframe the 6 January attack as a "beautiful day." He has referred to the individuals who participated as "patriots" and political prisoners, and vowed to pardon them if he returns to the White House.

Taking direct aim at this rhetoric, Mr Biden accused Mr Trump of trying to "steal history", attacking his rival by name repeatedly.

"Trump's mob wasn't a peaceful protest, it was a violent assault," Mr Biden said. "They were insurrectionists, not patriots. They were not there to uphold the Constitution, they were there to destroy the Constitution."

The Trump campaign did not immediately respond to a request for comment.

Jason Miller, a senior Trump campaign adviser, wrote on X that Mr Biden "has given up on running an issues-based campaign for 2024".

"Rather than help those suffering from Bidenomics or our porous southern border, Biden plans on weaponising government against his leading political opponent," Mr Miller wrote.

Mr Biden has returned to the theme of preserving democracy again and again.

In 2020, he campaigned as the candidate capable of restoring America to normalcy. Before the 2022 midterms, Mr Biden described that election as a "battle for the soul of this nation".

On Friday the venue in Valley Forge, Pennsylvania, a key site in the American Revolutionary War, was chosen to underscore the themes of Mr Biden's address.

Even the music contributed to the atmosphere: as Mr Biden strode to the podium, a song from Hamilton, a musical about the Founding Fathers, played over the loudspeakers.

After a year of failing to improve Mr Biden's standing in the polls by highlighting his economic agenda, his re-election campaign has re-focused on democracy, highlighting the stark differences between him and Mr Trump.

The strategy has been effective for Mr Biden before.

Democrats exceeded expectations in the 2022 midterms by hammering controversial, pro-Trump Republican opponents who questioned the legitimacy of US elections.

The issue "has incredible resonance with the Democratic base," said Ashley Etienne, a former Biden 2020 senior campaign adviser.

"There's nothing more foundational to who we are than democracy," Ms Etienne said. "But it is a winning strategy. It's a winning message. And it's facilitated this incredibly broad coalition of voters."

Mr Biden's Democratic allies seemed pleased with his decision to focus on democratic values.

"I'm glad to see the president going on the offence today to talk about democracy, to talk about freedom, and to talk about the fact that Trump puts all of that at risk," Pennsylvania Governor Josh Shapiro, a Democrat, told reporters.

Recent polling has shown that three years later, Americans' views of the 6 January attack are starkly divided along partisan lines, with Republicans now less likely to believe it was a violent attack. About a quarter of Americans believe a conspiracy theory that the FBI, not Mr Trump's supporters, instigated the riot, a Washington Post/University of Maryland poll found.

By Kayla Epstein & Gary O'Donoghue