Mitt Romney’s campaign and his super-PAC have begun ramping up their ad buys in Michigan to hit Rick Santorum where they believe he is weak — by painting him as an unreliable conservative, a big spender, and too much of a social conservative to win.  They need to undermine his credibility in a hurry if they want to reverse his momentum and bring Santorum back down to the second tier.  Santorum will have to find a way to defend himself while getting outspent, a task that Newt Gingrich couldn’t manage, and maintain his credibility as a national candidate.

In that, Santorum got some help yesterday from … Mitt Romney?

Mitt Romney, whose campaign is preparing a multi-million dollar wave of negative advertising to persuade voters that Rick Santorum should not be president, says he is open to the possibility of choosing Santorum to be his running mate should Romney win the Republican nomination.

Romney appeared on Fox News Wednesday morning and was asked, “You and Rick Santorum, we haven’t seen you go head-to-head yet…In the big picture, could you see a scenario where you two team up?”

“Oh, I think it’s always possible to have people come together in our party, whether it’s Rick and I, or others in the party, who knows?” Romney responded.  “It’s a little early to tell something like that, but we have similar views on issues — very different backgrounds.”

Romney’s “very different backgrounds” description of Santorum points to an argument Romney plans to use through the February 28 primaries in Michigan and Arizona and on through Super Tuesday on March 6.  Santorum has no executive experience — “hasn’t run anything,” Romney will say — and is not qualified to be president.  Romney, on the other hand, has run private businesses, the 2002 Olympics, and the state government in Massachusetts during his one term as governor.

Well, here’s the problem with that argument when made at the same time as suggesting the opponent as a valid running mate.  The position of VP/running mate exists for the sole explicit reason of replacing a President in a hurry, should the President die or become unable to carry out his/her duties while in office.  (Even the one official duty of the VP, President of the Senate, puts the VP in position to act as the President’s proxy.) If the “different backgrounds” issue is a disqualifier for the top of the ticket, it’s a disqualifier for the bottom of the ticket as well.

In that very real sense, Romney has essentially stepped on his own message.  At the same time he’s painting Santorum as unqualified for the Presidency, he’s trying to have it both ways and play to Santorum’s growing voter base by praising him in public by teasing out the possibility of a Romney/Santorum ticket.  Romney sees the need to do this because of Santorum’s high favorability ratings, whereas with Newt Gingrich he could just goad the former Speaker into a public fight and drive up his negatives even further.  However, this suggestion undermines the message Romney is using in Michigan, and it’s not a good moment for him to do that, considering his polling status in his native state.

Would Romney really consider Santorum for the bottom of the ticket? Possibly, but it wouldn’t make much sense for either man to have the other at the bottom of the ticket.  Romney wouldn’t accept it anyway, although Santorum might, but there are other options for both.  Santorum won’t gain anything by putting a Northeastern Republican on the bottom of the ticket (unless it were Chris Christie, perhaps); both would do better by picking a Tea Party favorite.  Bobby Jindal is probably the best choice for either, although Susana Martinez or Nikki Haley could work.