No. 45 sang the Dagestani fighter's praise during a live stream with Adin Ross on Monday ... and as soon as Notorious got wind of it, he ripped into Trump on X.

"Campaign ending decision," Conor said ... adding the right move would have been to deem an American fighter his favorite instead.

"I would have thought USA ONLY would be the mentality here and what I would want to see out of my president, personally."

Conor turning his back on Trump is rather interesting ... especially because he shared a strong message in his support after the assassination attempt last month.

"A 78-year-old multiple billionaire he should be on a yacht on the med touring golf courses," he said at the time. "But he is not. He is in Pennsylvania spitting out bullets! Running for the love of his country! God speed Donald! God bless the United States."

Conor not only changed his opinion on Trump -- he also walked back his "multiple billionaire" comment in Monday's post ... saying he was "actually wrong" and further stating he's worth just a "couple hundred million."

Of course, Conor's hated Khabib for years ... especially after he lost to the 29-0 fighter back in October 2018. He's taken every opportunity to rip his nemesis on social media ... and Trump's stance clearly rubbed him the wrong way.

Fellow UFC fighter Michael Chandler came to Trump's defense in the replies ... making a reference to their postponed fight over Conor's toe injury.

"If you had half the balls that DT had you would have shown up June 29th," Michael said on X. "Instead you’re normal walking, beer chuggin’ Austin 3:16 style at a local fight show/motorcycle rally in South Dakota, a RED, 100% #maga state by the way."

No word on what UFC honcho Dana White thinks about it all ... but we take it he's siding with Trump, as they're longtime buddies.