When President Barack Obama leaves the White House, he's taking something with him -- his six-pack(ish) abs ... according to his trainer who says El Jefe is still a workout freak.

Our photog in D.C. spotted Cornell McClellan, a member of the President's Council on Fitness, Sports & Nutrition -- and, more importantly, the man who keeps B.O. and the whole First Family in shape.

McClellan said it's not as tough as you'd think to bark out commands to the leader of the free world -- since, as he puts it ... Obama is "raging against death." Kinda strange for a guy who's only 53, but whatever.

As for the six-pack starter kit the Prez had right before winning office? It seemed to be fading in 2012, but McClellan explains why Obama's no lame duck in the gym.