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Just in time for Cinco de Mayo ...

Boxing superstar Adrien Broner says he's throwing his support behind Donald Trump -- and he doesn't care about pissing off Mexican boxing fans because they ALREADY hate him!!!!

Makes sense, Broner's been talking A LOT of crap about our neighbors to the south for years ... starting with the shot he took after a fight in 2014.

As for Trump, Broner appeared on "The Breakfast Club" and openly stumped for The Donald -- telling Charlamagne Tha God that he's voting for DT because he'll help save money on taxes!

And speaking of cash ... Broner was also asked if he REGRETS shooting down Jay Z's offer to join Roc Nation Sports last year ... you know, the one that came with a $40 MILLION CONTRACT!!

Check out the clip ... AB still thinks Jay's number is chump change -- but says he feels bad for blasting Rihanna at the time.