Ron Paul - Would get knocked out 1st round from age.
-probably been around the block a few times
Gingrich - I think he would put up somewhat of a fight because his weight would be like body armor but I think Obama could wear him out in 3 rounds. No KO though.
-slightly crazy(fuel to keep going)
-can't last long
Romney - This would be a great match up because Romney seems to be the only other opponent that seems to be somewhat fit. Private equity i'd imagine would create some pretty resilient people so I figure Obama wouldn't be able to out last him like Gingrich. I have no idea how his Mormon status would affect him in battle. Overall I give him 6 rounds of even fighting before Obama is able to knock him out.
-born in Detroit
-mentally strong(from cut throat industry)
-age(64 doesn't seem like it though)