Hmm. I thought you meant incumbent Obama, somehow able to go for a third term, he would have pretty decisively won.
From him just being senator for four years, hard to say but he would probably still win. They both would have been outsiders, Obama would try to tie Trump to Bush and wouldn't have been quite as successful as he was doing so with McCain but he would successfully make an issue out of Trump being way too rich and thus out-of-touch (a selfish plutocrat rather than a nationalist, not really for the U.S. at all just himself). Plus he would get less hostility from the left than did Hillary Clinton.