Rick Santorum was leading in most Michigan polls until a Romney negative ad blitz followed by a lackluster debate performance left him reeling and behind Romney in the most recent polls.

Romney’s main line of attack, seen in the ads and recited in the debate, focuses on Santorum’s time in in the House and Senate: especially voting for earmarks and debt ceiling increases and supporting Arlen Specter’s reelection in 2004.

One way of describing these attacks is to say Romney is criticizing Santorum for being a nationally relevant, relatively powerful Republican during the Bush years. After all, you only get to vote for earmarks and to raising the debt ceiling if you’re winning elections and serving in Congress.

As for supporting Arlen Specter, Santorum was following the lead of his party and the sitting Republican president to support a popular Republican in his home state. During Santorum’s years of supposed conservative heterodoxy , Romney was busy passing a universal health care plan .

What Romney is doing is taking advantage of the fact that, as a moderate Republican governor in a blue state, he wasn’t called on to take substantive political hits for the party. A Romney endorsement for Specter, for example, would have done next to nothing for Specter’s reelection. And had Romney won Teddy Kennedy’s seat in 1994 and been reelected to a second term, he probably would have been right there with Santorum and the overwhelming majority of Republicans, voting for earmarks and debt ceiling increases, supporting No Child Left Behind, and maybe even endorsing a moderate senator in a tough primary battle.

So, in a contest against someone who can stand in for the faults of Bush-Era Republicanism, Romney’s greatest asset is that he couldn’t win a Senate election and was too moderate as governor to be part of a Republican governance failure.