CHICAGO– United Automobile Workers (UAW) President Shawn Fain said he would not have spoken at the Republican National Convention (RNC) last month like fellow union leader Sean O'Brien after Vice President Kamala Harris' campaign apparently snubbed the Teamsters president this week.

Fain spoke with a small group of reporters on the sidelines of the Democratic National Convention (DNC) on Tuesday, the day after he gave an impassioned speech calling former President Trump a "scab" and advocating strongly for Harris's victory.

Meanwhile O'Brien told Fox News' Neil Cavuto in a televised interview on Tuesday that he has not yet heard from Democrats about his request to speak at the convention.

Fox News Digital asked Fain what he thought of the snub, and of O'Brien's decision to speak at Trump's convention last month — the first union boss in the RNC's 121-year history to do so. 

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Labor leaders in left-right photo split

UAW President Shawn Fein said he did not fault Teamsters President Sean O'Brien for addressing the RNC but said he himself would not have done so (Getty Images)

Fain stayed mum about his thoughts on Harris and Democrats ignoring O'Brien but said he did not fault the Teamsters leader on his decision — but said he did not see any use in doing so himself.

"Sean O'Brien has his reasons for doing what he's doing. I mean, you know, I think what he said is genuine, that he has members that he knows are going to vote that way. And you know, he's reaching out to both parties, I just — we have a difference in philosophy on that," Fain said.

"I mean, if the Republican Party gave me a reason to believe they cared about workers, we would, you know, entertain things like that. But I don't see any evidence of that."

Fain accused Republicans of voting against "every piece of legislation" in Congress "that pertains to making things better for working-class people."

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Teamsters union president speaking to RNC

O'Brien was the first union boss to address the RNC in its history (Andrew Caballero-Reynolds/AFP via Getty Images)

"So I don't know why I'd go stay in the Republican Convention and speak to that when they have no interest in it. I mean, if they start showing an interest in working with people, it's one thing, but they don't. And so, you know, I don't, I don't believe that. I want our members to understand there's not any doubt where we stand in this election."

Fain added that "we don't tell people how to vote," but "it's our job to put the facts in front of you and give you the proper information about what's best for labor."

And Fain did make it clear where he stood during his Monday night speech, rousing the supportive crowd into chants calling Trump a "scab." A UAW spokesperson told Fox News Digital that the union has sold "thousands" of shirts with the phrase since Fain's remarks.

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Fox News Digital reached out to the Teamsters for comments on his latest remarks.

O'Brien told Fox News Channel host Neil Cavuto on Tuesday that the Harris campaign agreed to a roundtable with the Teamsters general executive board and with rank-and-file members — but said he was not taking their silence on the DNC to heart.

Shawn Fain speaking to the DNC in 'Trump Is a Scab' t-shirt

UAW said it sold "thousands" of shirts calling Trump a "scab" since Fain's DNC speech (REUTERS/Mike Segar)

"I could take it personally, but I honestly think maybe my invitation got lost in the U.S. Postal Service. And next time, they should try and ship a little guaranteed delivery," O'Brien said. 

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He also defended his appearance at the RNC, telling Fox News Channel, "Look, the the DNC failed to invite one of the most powerful unions in the country there. And, you know, I'm going to criticize them for that as well."

'I'm a lifelong Democrat, always will be a lifelong Democrat, and never going to change my party affiliation. Look, you heard my speech at the RNC. It was all about the American worker, all about labor. Any opportunity that I get as a leader of the greatest union in the world to showcase our value and our work to the greatest country in the world. I'm going to take that opportunity to do it. And if the far left want to criticize me and attack me, that's okay. If the far right wants to do the same. That's great," O'Brien said.