Sen. John Fetterman had a major slip of the tongue … as the centrist Democrat — who often splits from his party — called himself a “lifelong Republican.”

Fetterman made the gaffe while he was attending the Texas Tribune Festival Saturday … and the moderator had to remind him that he was in fact a Dem.

Fetterman — wearing his classic sweatshirt and shorts uniform — could be seen putting his hand to his head after the viral moment.

He admitted of the mistake … “I guarantee you that will break through. That will get clipped.”

What can we say, John … when you’re right, you’re far right. Kidding!

Despite the error … he firmly responded that he doesn’t “have a reason” to join the GOP — a party he crossed the aisle to side with around 26% of the time in 2025, according to voting stats.

At the same time, Fetterman said … “If the Democratic Party votes me out or expels me, they’re always entitled to do that.”