Valley Forge, Pennsylvania - President Joe Biden will try to fire up his 2024 campaign on Friday with a major speech warning that democracy is at risk from Donald Trump, three years after the January 6 US Capitol attack.

Either trailing or neck-and-neck with Trump in recent polls, the 81-year-old Democrat will frame his likely Republican rival as a threat to the nation in an address near the historic Revolutionary War site of Valley Forge in Pennsylvania.

A looming winter storm forced the speech to be brought forward a day from Saturday, the third anniversary of the Capitol assault by a pro-Trump mob trying to overturn Biden's 2020 election win.

The effort to boost Biden's faltering campaign by painting him as a defender of democracy will continue Monday when he visits a South Carolina church where a white supremacist fatally shot nine Black parishioners in 2015.

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Campaign manager Julie Chavez Rodriguez said Biden's original election pitch, that he was leading a "battle for the soul of America" four years ago, has become more relevant than ever. "The threat Donald Trump posed in 2020 to American democracy has only grown more dire in the years since," she said in a statement.

The venues for Biden's first speeches of 2024 are deliberately symbolic – especially the first, at a school near Valley Forge.

"We chose Valley Forge as George Washington united the colonies there," said principal deputy campaign manager Quentin Fulks.

"Then he became president and set the precedent for the peaceful transition of power – something that Donald Trump and Republicans refused to do."

This increased push at the beginning of 2024 comes after criticism from some Democrats that the Biden campaign has got off to a slow start.

Biden has failed to convince voters that the economy is improving despite favorable numbers with Americans saying they are still suffering from high food and housing costs.

Migration across the Mexican border also remains a major headache. There is additional division in his party over his support for Israel's war on Hamas and Congress blocking his bid for funding to Ukraine.

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Biden's refusal to mention Trump's multiple criminal cases, potentially to avoid the appearance of influencing the judiciary, has also deprived him of one of his most potent weapons.

But perhaps Biden's biggest vulnerability is his age: as America's oldest-ever president, he has suffered a series of trips and verbal slips.

Biden lags behind Trump in a series of polls and also has the worst approval rating of any modern president in the December before an election. Recent speeches show the Biden campaign will now portray the race as a straight choice between him and the twice-impeached former president.

His campaign is already treating Trump as the presumptive challenger despite the fact that the battle for the Republican nomination doesn't even get underway until the Iowa caucuses on January 15.

Meanwhile, Biden's first TV ad of the year warns of an "extremist" threat to democracy, featuring images of the Capitol attack and dramatic music.

"It was a sight that was horrific," White House Press Secretary Karine Jean-Pierre told reporters on Thursday. "The president is going to continue to speak about this and continue to be very vocal about this."

Democrats are also targeting Trump on issues such as abortion access and healthcare.