Blue Bell, Pennsylvania - President Joe Biden accused Donald Trump of echoing Nazi Germany as he kickstarted his 2024 reelection campaign Friday with a major speech warning of a threat to democracy.

"He's willing to sacrifice our democracy, put himself in power," the 81-year-old Democrat said of his likely Republican rival on the eve of the third anniversary of the January 6 Capitol attack.

Biden chose a symbolic location for the speech near Valley Forge, the historic site where George Washington regrouped American forces during the War of Independence nearly 250 years ago.

He said that twice-impeached former President Trump had failed to prevent the Capitol mob assault in 2021 and accused the tycoon and his supporters of still embracing political violence ahead of the 2024 vote.

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"He talks about the blood of Americans being poisoned, echoing the same exact language used in Nazi Germany," said Biden, who was greeted by chants from supporters of "four more years."

"Donald Trump's campaign is obsessed with the past, not the future. He's willing to sacrifice our democracy, put himself in power," Biden said in the speech in the battleground state of Pennsylvania.

Biden's address marked an aggressive start to the year as he either trails or is neck and neck with Trump – the man he beat in 2020 – in recent polls.

Trump was impeached but acquitted over the January 6 riots. The 77-year-old now faces a criminal trial on charges of trying to subvert the 2020 election.

Colorado and Maine have also barred him from standing in presidential primaries on the grounds that he had engaged in insurrection over the Capitol events. Trump has challenged both rulings.

Trump spokesperson Steven Cheung immediately reacted to the speech, saying that "Biden is the real threat to democracy by weaponizing the government to go after his main political opponent and interfering in the 2024 election."

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At Valley Forge, where Washington rallied American forces fighting their British colonial rulers during the bitter winter of 1777-8, Biden attended a wreath-laying session and visited a cottage used by America's first president.

Biden's speech was scheduled for Saturday but brought forward by a day because of a looming winter storm.

The president's attack on Trump came after criticism from some Democrats that the Biden campaign has gotten off to a slow start.

Biden lags behind Trump in some polls and also has the worst approval rating of any modern president at this stage in his term of office.

The president has failed to convince voters the economy is improving. Despite further job growth in December, he acknowledged Friday in a statement that "some prices are still too high for too many Americans."

Migration remains a major headache, while there is division in his party over his support for Israel's war on Hamas, and Congress is blocking his bid for more funds for Ukraine.

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.

"If the election were held tomorrow, President Biden would lose," William Galston, senior fellow at the Brookings Institution, told AFP.

Yet the Pennsylvania speech showed the Biden campaign is now playing up a straight choice between him and Trump, even though the battle for the Republican nomination doesn't even start until the Iowa caucuses on January 15.