Vandalia, Ohio - Donald Trump told a rally in Ohio on Saturday that November's presidential election will be the "most important date" in US history, painting his campaign for the White House as a turning point for the country.

Days after securing his position as the presumptive Republican nominee, the former president also warned of a "bloodbath" if he is not elected – though it was not clear what he was referring to, with the remark coming in the middle of comments about threats to the US auto industry.

"The date – remember this, November 5 – I believe it's going to be the most important date in the history of our country," the 77-year-old told rally-goers in Vandalia, Ohio, repeating criticisms that his rival, President Joe Biden, is the "worst" president.

Criticizing what he said were Chinese plans to build cars in Mexico and sell them to Americans, he stated: "We're going to put a 100% tariff on every single car that comes across the line, and you're not going to be able to sell those cars if I get elected."

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"Now if I don't get elected it's going to be a bloodbath for the whole – that's going to be the least of it, it's going to be a bloodbath for the country. That'll be the least of it. But they're not going to sell those cars."

As Trump's comment gained traction on social media, Biden's campaign released a statement calling the Republican a "loser" at the ballot box in 2020 who then "doubles down on his threats of political violence."

"He wants another January 6 but the American people are going to give him another electoral defeat this November because they continue to reject his extremism, his affection for violence, and his thirst for revenge," the campaign said, referring to the deadly attack on the US Capitol by Trump supporters in 2021.

Later, Biden spoke at a dinner in Washington, where he also warned of "an unprecedented moment in history."

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"Freedom is under assault... The lies about the 2020 election, the plot to overturn it, to embrace the January 6 insurrection pose the greatest threat to our democracy since the American Civil War," he said.

"In 2020, they failed, but ... the threat remains."

The 81-year-old, who has waved off concerns that he is too old for a second term.

"One candidate's too old and mentally unfit to be president," he said of the presidential race. "The other guy's me."

Earlier this month, Trump and Biden each won enough delegates to clinch their party nominations in the 2024 presidential race, all but assuring a rematch and setting up one of the longest election campaigns in US history.

Among the issues Trump is campaigning on is sweeping reform of what he calls Biden's "horror show" immigration policies. The ex-president successfully pressured Republicans to block a bill in Congress that included the toughest border measures in decades, described as dangerous and inhumane by rights advocates.

On Saturday, he invoked the border again as he reached out to minorities who have traditionally voted Democrat.

He said Biden had "repeatedly stabbed African-American voters in the back" by granting work permits to "millions" of immigrants, warning that they and Hispanic Americans "are going to be the ones that suffer the most."

For decades Ohio had been seen as a bellwether battleground state, though it has trended more strongly Republican since Trump's White House win in 2016.

The rally came a day after Trump's former vice president, Mike Pence, said he would not endorse his old boss for a second White House term.