WASHINGTON (Reuters) - The president of the International Brotherhood of Teamsters, will speak at the Republican National Convention (RNC) next month in Milwaukee, former President Donald Trump said in a post on his Truth Social platform on Friday.

Trump said the Teamsters president, Sean O'Brien, had "accepted my invitation to speak at the RNC Convention in Milwaukee."

The 1.3 million-member influential labor union has yet to endorse a candidate in the presidential election. It endorsed Democratic President Joe Biden in 2020.

Kara Deniz, a spokesperson for the union, said O'Brien asked to speak at both the RNC and the Democratic National Convention, which will be held in Chicago in August.

"The Teamsters are excited that former President Trump has extended this invitation, and we will be equally excited if and when the DNC chooses to do the same," she said.

It would be unusual in the current polarized political climate for an individual to speak at both conventions.

Earlier this year, O'Brien met both Biden and Trump, and said the union would conduct a lot of polling to make a decision on its endorsement, "most likely after the (Republican and Democratic) conventions."

Trump and Biden are both courting votes from rank-and-file members of organized labor, whose support in the Nov. 5 election could be crucial in battleground states such as Michigan, Wisconsin and Pennsylvania.

Biden, who has described himself as the most labor-friendly president in history and who turned up at a picket line in Michigan during the autoworkers strike last fall, has already received significant organized labor backing with early endorsements from the AFL-CIO and the United Auto Workers union.

Trump has been trying to make inroads into Biden's support among organized labor heading into the general election, as he works to win over more blue-collar workers, who helped fuel his 2016 victory.

The Teamsters is one of the country's largest unions and represents truck drivers, dockworkers, airline pilots, government employees and many other sectors.

(Reporting by Nandita Bose in Washington; Editing by Leslie Adler)

By Nandita Bose