The former prez called into "Fox & Friends" after the debate and Swift's highly-publicized endorsement ... and while 45 tried to seem calm and cool about the whole thing, he was a bit miffed by her decision.

As Trump put it ... He didn't even want Swift's endorsement to begin with, since he wasn't even a "Taylor Swift fan."

He added ... "Well, I actually like Mrs. Mahomes much better."

Brittany Mahomes, the wife of Kansas City Chiefs quarterback Patrick Mahomes and Swift's good friend, is among the list of celebrities who've spoken out in support of Trump amid his 2024 reelection bid.

It's clear Trump prefers Mahomes to Swift, referring to the former soccer player as "a big Trump fan."

Trump couldn't help but take a shot at Swift, however ... suggesting the pop sensation would "probably pay a price for [endorsing Harris] in the marketplace."

While we're sure some of her fans are upset by her decision, it's no secret Swift leans Democrat ... having endorsed President Joe Biden over Trump 4 years ago. Swift went on to have the biggest tour of her career and drop several chart-topping albums.

Trump was also well aware that Swift may endorse Harris ahead of the election, having previously called her "liberal" over the summer.

Still, Trump tried to rally Swifties to his side by sharing an AI image of her appearing to endorse him back in August.

Swift -- who has been a victim of inappropriate AI imagery before -- called out the fake endorsement in her letter supporting Harris' campaign.

Elon Musk has also weighed in on Swift's endorsement decision ... taking to X and promising to give her "a child" and protect her cats with his life.

The tech giant's response was a clear dig at Swift's signature on her endorsement note, which she signed off as a "childless cat lady" -- a reference to Trump's running mate J.D. Vance's controversial past comments.