At this point...

[Stranger in a strange land]

Portman's approval ratings are lower than Obama's in OH, and the fact that he had worked for George W. Bush would turn into a significant liability. Portman would help Romney by a point or two in Ohio, but probably not by enough to win the state, leading to a result something like this:

Obama 332
Romney 206

The margin is a bit closer than in 2008, but not by much and not because of Portman.