There's no provision in the requirement to become Commander in Chief that one serves first. We've had several Presidents who didn't serve and sent men and women off to battle.
I do understand though that the dems are really gearing up for the election and all is fair in politics and war. Call him the "liar", the "draft dodger", "the phony", it all fits. He'll still win because no one on the other side isn't all of those things and more.