A Donald Trump protester -- who happens to be black -- got knocked to his ass by a white Trump supporter, and now cops have arrested the attacker ... TMZ has learned.

The video from the Wednesday rally in Fayetteville, NC shows an unidentified supporter in a cowboy hat totally sucker punch a protester who was already being escorted out of the venue.

Officers, who observed the hit, reacted quickly, but slammed the protester down on the ground -- not the puncher.

We've learned police did not arrest the victim, but did kick him out of the event. As for the punching cowboy -- we've learned he's 78-year-old John McGraw, and he's currently in police custody. According to cops, McGraw voluntarily came in Thursday morning to be interviewed ... and that's when he was arrested.

We're told he's being booked for assault and disorderly conduct. He's scheduled to appear in court April 6.