At least one voting block is already behind Mark Cuban's possible 2020 presidential bid -- his players ... with Mavs vet, Devin Harris, telling TMZ Sports a Cuban country might not be a bad idea.

Cuban hasn't officially thrown his hat into the ring yet, but appeared on "OBJECTified" and told Harvey Levin he's considering challenging Donald Trump as a republican next go-around.

Harris has spent much of his 13-year NBA career playing under Cuban, and when we got him out at Warwick in L.A. earlier this week, we had to ask if he's down with M.C. taking his talents to D.C.