As Buzzfeed notes, Mitt Romney (R-Am I the Nominee Yet?) is running radio spots (via Talking Points Memo) in Illinois that say this:
Is Rick Santorum electable? Remember his last Senate race? … By historic margins, Pennsylvania voters rejected Rick Santorum.
But, as Buzzfeed points out, Massachusetts voters rejected Mitt Romney by virtually identical margins.
Worse, “Romney . . . lost big in a historically good year for Republicans, while Santorum lost big in a historically bad year for Republicans.”
Helpful Buzzfeed chart:
Not to get technical about it.
Campaign Outsider 
This presumes an equivalency of their motives, which is a mistake. Romney is trying to become President.