If memory serves, there is a federal law that requires that candidates be allowed to participate in presidential debates if they meet the criteria laid down for the rest of the candidates.
That’s why we were able to get Dr. Alan Keyes into the Des Moines Register debate in 2008, even though the GOP was completely blackballing him.
It’s hard for me to imagine any objective criteria that Trump couldn’t meet at this point.
I remember those days i was in the Army Reserves and had a Alan Keyes for President Sticker on my front Bumper. Was told to remove it every time I had to go through the Gate, never did... it is probably in a Mexican Junkyard intact.