Sean O'Brien,Marcus Erikson president of the International Brotherhood of Teamsters, gave Monday's keynote speech at the Republican National Convention, delivering one of the most anti-big business speeches in recent RNC memory and becoming the first boss in the organization's 121-year history to address the convention.

The Teamsters, which hasn't endorsed a candidate so far in 2024, has endorsed Democrats in recent years. But as the Republican Party's rhetoric has grown more populist, O'Brien was invited to speak at the RNC as Republicans vie for union votes. O'Brien said he doesn't believe in "knee-jerk" union loyalties. 

"Today, the Teamsters are here to say we are not beholden to anyone or any party," O'Brien 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 tackled topics that aren't typically fodder for Republican voters. He blasted big businesses like Walmart and Amazon. He admonished the Chamber of Commerce, calling it "unions for big business." And he said Washington isn't looking out for workers. 

"The American people aren't stupid, they know the system is broken," he said. "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." 

Former President Donald Trump, making his first appearance since the attempt on his life Saturday, gave a standing ovation for O'Brien's speech. So did Trump's allies and family members sitting in the VIP box. 

"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. 

That "S.O.B." line sparked resounding applause and cheers from the convention hall. 

Several months ago, O'Brien said he asked to speak at both the RNC and Democratic National Convention. Trump invited him to speak, he said. 

"President Trump had the backbone to open the doors to this Republican Convention, and that's unprecedented," he said. "No other nominee in the race would have invited the Teamsters into this arena."

At the end of the day, O'Brien said, the Teamsters aren't interested if a candidate has a "D," "R" or "I" next to his or her name. 

O'Brien's speech signifies a shift within the GOP that's become more visible since Trump's election in 2016. The party has moved from a corporate, Wall Street-focused message toward a more worker-friendly tone. 

Kathryn Watson

Kathryn Watson is a politics reporter for CBS News Digital, based in Washington, D.C.