Newton, Iowa – Donald Trump hit the campaign trail Saturday in the first-in-the-nation presidential nominating state of Iowa, vowing to win "for the third time" as Americans marked the anniversary of the deadly assault on the US Capitol by a mob of his supporters.

Many watched in horror three years ago on January 6, 2021, as rioters – egged on by the ex-president and fueled by his false claims of voter fraud – stormed the seat of US democracy in a bid to halt the transfer of power.

In a rambling, two-hour speech Saturday to supporters in Newton, Iowa, Trump did not delve into the events of that day, but described those jailed over their roles in the assault as "hostages." He has said that if elected, he will pardon many.

Trump again mocked President Joe Biden, his putative rival in November's election.

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He said Biden has overseen economic decline and invited chaos at the nation's borders, while failing to stop Russia's invasion of Ukraine.

"I would've absolutely stopped Putin," he said.

Trump warned of World War III if Biden is re-elected, adding, "This is our last chance to save America."

Never having given up on the fiction that he won the 2020 election, Trump declared that in November he will win "for the third time."

The runaway leader in the race for the 2024 Republican presidential nomination, Trump was impeached for inciting insurrection and faces multiple felony charges over his conduct leading up to and during the violence.

In an evening appearance at a middle school in the town of Clinton, Trump gave another meandering speech on Saturday in which he attacked Biden as too old to govern, incompetent, and leading the United States to ruin with overrun open borders.

"We are a failing nation," Trump said in the hour-long address.

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"We're going to bring it back from hell," he added.

Biden, who offered blistering criticism of Trump in a speech Friday, has no public events planned this weekend, the White House said.

Trump leads rivals Nikki Haley and Ron DeSantis by more than 30 points in Iowa, which stages its Republican nominating contest – known as a "caucus" – on January 15, kicking off the 2024 primary season.

Trump has described January 6 as "a beautiful day" and has made the "great patriots" and "hostages" imprisoned over the riot a cause célèbre.

The false claim that Democratic election theft led to January 6 has become orthodoxy among many Republicans, with hard-liners in Congress promoting the fantasy that the chaos at the Capitol was a "false flag" operation by federal agents. A Washington Post/University of Maryland poll released Thursday found that 44% of Trump voters believe the FBI organized and encouraged the riot.

But polls also show that Democrats and independents hold strongly negative views of the riot and the rioters, and of Trump's role in encouraging them.

"We will never forget the horrific events of January 6, 2021," Democratic House Minority Leader Hakeem Jeffries said in a statement Saturday, adding that those responsible "must be held accountable, because in this country no one is above the law."

Vice President Kamala Harris posted on social media that a victory by those "who seek to dismantle our democracy" would bring violence and chaos.