The Utah U.S. Senator showed face Saturday at a GOP convention at the Maverik Center in his home state -- but not one of the 2,000 or so delegates on hand seemed all too happy to see him step to the podium ... and they made their displeasure known, very loudly.

Check out this brutal scene ... the entire building is straight-up booing the man, with no shame whatsoever. He was also reportedly met with taunts and jeers, likening him to a "communist" and a "traitor." Unfortunately for Romney, he got booed the hell off stage.

We'll give him credit -- he tried holding his own up there for a while, even bringing up Biden's first 100 days, seemingly in an effort to curry favor with the Utah crowd. They weren't having any of it, though ... and Romney eventually addressed the elephant in the room.

He said it's no secret he wasn't the biggest fan of Trump's "character" before someone came to get him. That's an understatement if we ever heard one -- and big-time downplaying.

Fact is ... Romney drew his line in the sand many times over, especially with his impeachment votes. You might recall ... he twice voted to convict Trump at his trials, agreeing that the guy had either 1) abused his presidential power or 2) incited an insurrection. Trump was acquitted in both instances.