WASHINGTON - Donald Trump will refuse to accept the results if he loses the US election this year, Joe Biden said Wednesday.

The two men, who fought for the White House in 2020, are set for a rematch in November that polls show the American public is unenthusiastic about.

In an interview with CNN, Biden raised the prospect of Trump not accepting the outcome of the ballot. "I promise you he won't, which is dangerous."

Trump still insists — without foundation — that he won the 2020 election, despite losing repeated legal challenges.

"How many court cases did they have? Supreme Court cases? They all said 'this is a totally legitimate election,'" said Biden. "This is Trump."

"You can't only love your country when you win."

Biden was speaking in the key battleground state of Wisconsin, a state Trump was also campaigning in last week.

The former reality TV star, who is currently embroiled in a court case over alleged hush money payments to porn star Stormy Daniels, last week hinted to local media that he might dispute the 2024 election.

Trump told the Milwaukee Journal Sentinel he would accept the result "if everything's honest."

"If it's not, you have to fight for the right of the country," he told the paper.

The 81-year-old Biden, who is already the oldest person ever to hold presidential office in the United States, said US allies were hoping he would win a second term in November.

"Look, I travel around the world; other world leaders, you know what they all say? Not a joke.

"Eighty percent, after we have a major meeting (say) 'You gotta win... my democracy's at stake.'"

Biden said comments Trump has made on the campaign trail demonstrate how he will bend the presidency to his own ends if he wins back the White House.

"He's saying... he's gonna make sure his attorney general prosecutes those he tells him to prosecute and if he doesn't, he's going to fire him.

"He said this about prosecution across the board. 'I'm your retribution.' What president has ever said anything like this stuff? But he means it."

Biden, who is lagging his expected rival in many opinion polls, said he was not concerned, noting the election was still many months away.

"I think I'm feeling good about the trajectory of the campaign," he said.

"And you know as well I do, most people don't really focus and make up their minds until the fall. There's a lot going on, and we'll see what happens."

Asked what advice his former boss, President Barack Obama, had provided in their conversations about the race, Biden said it was simply to "keep doing what I'm doing."