If the Romney family is invested in at least one of the firms that make voting machines, a claim that they were rigged in the event of a Romney loss would be an admission of incompetent oversight at that firm.
If it turns out that a few thousand votes going the other way in one or two states would have given an Electoral College victory to Romney, there’s definitely going to be fraud claims. Enough that obstructionism won’t seriously hurt Republicans in Congress (because if he’s not “really” President, refusing to work with him is patriotic).
Liberal media.
Thi, as evidenced by a joke that himself made at the Al Smith dinner: “The media have their job, and I have mine; my job is to lay out a clear, concise plan for America’s future, and their job is to make sure you never hear about it.”*
I know it’s a gag, but the if there was such a plan, they might consider making it available at mittromney.com, or something crazy like that - and joke or not, it will become the official line in the event of a defeat.
*paraphrased, because I can’t be arsed to find the exact quote.