Basically Romney's challenge if he loses PA and OH is to win every single other swing state (FL, NC, VA, NH, CO, NV and IA). This gives him 273. It is possible, but odds are tough.
The 269 tie of losing OH and NH is also a possibility. And maybe the Romney campaign could pick off ME-2 at the end to avoid the hassle (remote).
I really don't see him breaking into any of the next tier of Dem states (PA, NM, WI, MN, MI, OR) while still losing OH and one of the other more swingy states.
Bottom line is that Romney needs to turn things around in OH. If he picks Portman as VP, it would indicate that they also see OH as a problem that needs to be fixed (not that he is going to be able to fix it easily)