I don't think this is all because of Strickland. Portman's not exactly unpopular, and he's a relatively inoffensive senator. I always had a hard time believing that he was in as much trouble as earlier polls suggested. I initially thought he'd be less vulnerable than Toomey or Rubio, and it looks like I was probably right about that. Strickland's not totally done yet, but this race is at at least Lean R, by this point.
It has nothing to do with Portman (who isn't very entrenched). Strickland has run an awful campaign, he'll still do much better than Sittenfield would've though.