A labor leader who almost came to blows with a Republican U.S. senator is set to address GOP delegates at the party's presidential nominating convention next month.

Sean O'Brien of the International Brotherhood of Teamsters will be the group's first president to speak at a Republican National Convention, a union spokesperson said Friday, according to reports.

"Our 1.3 million members represent every political background, and their message needs to be heard by as wide an audience as possible, and that includes all political candidates running for elected office," said Teamsters spokesperson Kara Deniz.

Deniz also said the union appreciated former President Donald Trump's "openness to inviting a labor leader to speak on behalf of working families."

O'Brien asked to speak at both the Republican and Democratic conventions, but hasn't yet heard back from the Democrats, Deniz said.

The Republican National Convention is set to start on July 15 in Milwaukee, just days after Trump is scheduled to be sentenced in New York City for his conviction last month in the Stormy Daniels "hush money" case.

During a congressional hearing in November, O'Brien was challenged to a fight by Republican Sen. Markwayne Mullin of Oklahoma, who read aloud a series of social media messages in which O'Brien called him a "moron" and said he was "full of s---."

"If you want to run your mouth, we can be two consenting adults. We can finish it here," Mullin said.

The men exchanged insults for several minutes, ignoring repeated orders to stop talking from independent Vermont Sen. Bernie Sanders, who was chairing the meeting,

They didn't stop until Democratic Sen. Maggie Hassan of New Hampshire began her questioning by talking over them.

The influential Teamsters union endorsed President Joe Biden in his 2020 race against Trump, but hasn't yet announced its choice for the November election.

In a Friday post on his social media website, Trump said: "When I am back in the White House, the hardworking Teamsters, and all working Americans, will once again have a country they can afford to live in and be respected around the world."

Biden has repeatedly described himself as the "most pro-union president in American history" and in September became the first president to participate in a picket line when he joined striking members of the United Auto Workers union in Belleville, Michigan.

The UAW endorsed Biden in January, with president Shawn Fain saying: "If Donald Trump ever worked in an auto plant, he wouldn't be a UAW member. He'd be a company man, trying to squeeze the American worker."