Donald Trump told a rally in Ohio 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 on Saturday in Vandalia, Ohio, repeating well-worn 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: "They're not going to be able to sell those cars if I get elected.

"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."

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.

Reached out to minorities

Among the issues, Trump is campaigning on is sweeping reform of what he calls Biden's "horror show" immigration policies, despite successfully pressuring Republicans to block a bill in Congress that included the toughest border security measures in decades.

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.