The former prez took to Truth Social early Sunday morning and addressed the world's biggest celebrity couple -- as well as rumblings that Taylor would endorse Biden again.

DT started by touting one of his legislative accomplishments ... passing the Music Modernization Act -- a law that basically changed the way songwriters and music publishers are paid certain royalties -- on the whole, it helps artists receive fairer compensation.

The MMA was signed into law in 2018, under 45 ... so he feels he helped make Swift some serious cash -- adding Joe Biden hasn't done "anything for Taylor, and never will."

He then invoked the word "disloyal" ... saying Swift could never turn against "the man who made her so much money."

Trump finished off his post with some complimentary words for Travis Kelce ... saying he quite likes the two-time Super Bowl champ -- even if he is a liberal!

While not outright complimentary of Swift, this seems like a shift away from his campaign's reported strategy regarding the world's biggest pop star.

Remember ... Rolling Stone reported the Trump campaign was preparing for a "holy war" against her. Unclear exactly what that means, but this isn't the blatant condemnation one would expect from holy warriors.

It does seem to draw a line in the sand though ... ex-POTUS expects loyalty -- and it seems like the situation could get messy if T-Swift doesn't kiss the ring.

Taylor endorsed Joe for his 2020 presidential run ... and it seems unlikely she'll back Trump this time, though we suppose anything is possible.

Kickoff is just a few hours away ... but a much bigger battle seems to be brewing on the horizon.