Teamsters’ boss Sean O’Brien’s speech at the Republican National Convention comes as a tacit endorsement of Donald Trump and a major blow to Joe Biden and Democrats.

O’Brien, who was given a prime time speaking role Monday in Milwaukee, called the former president a “tough S.O.B.” and is the first Teamsters leader to speak at the Republican gathering in history.

The union has yet to endorse a candidate in 2024 and may not this year, but Biden has sought to court big labor and counts himself as a supporter of unions.

But O’Brien’s appearance at the RNC sends a signal to union members that voting for Trump is okay.

“President Trump is a candidate who is not afraid of hearing from new, loud and often critical voices, and I think we all can agree, whether people like him or they don’t like him, in light of what happened to him on Saturday, he has proven to be one tough S.O.B.,” O’Brien said of the assassination attempt on Trump.

Delegates cheered the line wildly and Trump himself joined in giving O’Brien a standing ovation.

The union boss’s speech raised memories of the Boston Police Patrolmen’s union backing George H.W. Bush over Gov. Michael Dukakis in the 1988 presidential race. The endorsement – which Bush personally accepted in Boston – was a major embarrassment for the Democratic governor.

O’Brien said on Monday he didn’t believe in “knee-jerk loyalties” – explaining why the union hasn’t endorsed Biden yet.

“Today, the Teamsters are here to say we are not beholden to anyone or any party,” he said. “We will create an agenda and work with a bipartisan coalition, ready to accomplish something real for the American worker. And I don’t care about getting criticized.”

O’Brien’s controversial speaking invitation is part of the Republican Party’s attempt to reach out to blue-collar voters, especially in the Rust Belt, who voted for Biden in 2020 but may now shift back to Trump.

“President Trump had the backbone to open the doors to this Republican convention, and that’s unprecedented,” O’Brien said. “No other nominee in the race would have invited the Teamsters into this arena.”

In his speech, O’Brien also blasted big businesses Walmart and Amazon – something that left-wing Sen. Elizabeth Warren usually does – and the national Chamber of Commerce.

“The American people aren’t stupid, they know the system is broken. We all know how Washington is run. Working people have no chance of winning this fight. That’s why I’m here today, because I refuse to keep doing the same things my predecessors did,” he said.

O’Brien has also requested a speaking slot at the Democratic Party convention next month.