Washington DC - Former US president Donald Trump met leaders of one of the country's largest labor unions on Wednesday as he seeks the backing of blue-collar America in his bid for a White House return.

The sit-down with the International Brotherhood of Teamsters in Washington came with Trump, a Republican, hoping to leach away support for current Democratic President Joe Biden among manual laborers as the pair look set for a rematch in November's election.

Republicans are the less labor-friendly of the two parties, but many of their conservative social policies have been able to resonate with rank-and-file union members at the ballot box even as labor leaders have generally endorsed Democrats.

"A big part of the voting bloc votes for me – a very big part... nobody knows what the exact number is – but some people say more than 50% of the Teamsters vote for me," Trump told reporters after the meeting.

"So we had a very productive meeting with a lot of Teamster representatives."

Biden – sometimes nicknamed "Union Joe" – was endorsed last week by the United Auto Workers (UAW) and describes himself as the most pro-union US president ever.

That backing earned the UAW president Shawn Fain an angry rebuke from Trump, who called the labor leader a "dope."

Trump is leading former South Carolina governor Nikki Haley comfortably after the two opening contests in the Republican primary and is expected to be anointed as the party's standard-bearer at its convention in July.

But victory in November will require him to win back much of the support he hemorrhaged to Biden in 2020 in the all-important swing states of the Midwest – the working-class heartland of American manufacturing.

Trump is not expecting to receive the Teamsters' endorsement, as the union went with Biden in 2020, praising the Democrat's "long history of standing with unions and workers on the job site."

"Stranger things have happened. Usually a Republican wouldn't get that endorsement – usually they only do Democrats," Trump said.

"But my case is different as I've employed thousands of Teamsters."

Teamsters president Sean O'Brien described the conversation as "pleasant" and "direct," telling reporters Trump and a group of officials discussed workers' rights, bankruptcy reform, and other union priorities.

O'Brien added that Biden has also committed to a meeting and his members had "more questions" of both candidates.

He said his Teamsters would typically reveal who they are backing after the summer party conventions, but added that "the endorsement or non-endorsement may come sooner than later. We haven't made a decision."