They can't say, "Don't vote for Obama because he's a black guy." But note how close R-Money comes to saying "Vote for me because I'm a white guy" in this commercial:
“...But there’s something more than legislation or new policy. It’s the feeling we’ll have that our country’s back, back on the right track. That’s what will be different about a Romney presidency.”
And that's why Mitt doesn't want to run on his public sector record, or his private sector record, or his religion, or even his policies. Mitt is running on the promise of restoring the natural order of politics: with him as president, you will be able feel better about yourself and America's future because it will be a WHITE guy who is screwing you over. And if he can convince enough independent voters of that, he can win. Surely a lack of change from a white president is superior to a lack of change from a black one!