Rick Santorum will be the third presidential candidate to enjoy the endorsement of Ohio Attorney General and former U.S. Sen. Mike DeWine. The AG, who was the force behind Ohio’s participation in the states’ lawsuit to block Obamacare, initially supported former Minnesota Gov. Tim Pawlenty. When Pawlenty exited the race, DeWine transferred his support to Mitt Romney, whom Pawlenty also backed. Today, though, DeWine will reassign his endorsement yet again — this time to his former colleague in the Senate, Rick Santorum, according to Associated Press sources.

Two ways to look at this: Either (a) the endorsement means more because DeWine once liked Romney, signifying that he’s not just supporting Santorum because he can’t stand Romney or (b) the endorsement means less because DeWine once liked Romney, signifying that he just can’t make up his mind and might not actually be 100 percent sold on Santorum, either.

Because I like Santorum, I’ll interpret this positively for the former Pennsylvania senator. DeWine’s defection not only suggests a certain inadequacy in Mitt Romney, but also suggests he sees enough presidential potential in Santorum to make it worth his while to publicly change his opinion.

Until DeWine makes his official announcement and explains why he chose to make the switch, we won’t know for sure what led him to change his mind, but Romney supporter and former White House Chief of Staff John Sununu speculates it had to do with an ad by the pro-Mitt Romney Super PAC, “Restore Our Future.”

“It is my understanding that the Super PAC that supports Gov. Romney had an ad out there that mentioned that Rick Santorum supported this amendment that gives voting rights to convicted felons and that the only three Republicans who voted for that amendment were Arlen Specter, Rick Santorum and Mike DeWine,” Sununu said on a conference call. “We have had people tell us that Mike DeWine was upset that that issue was raised by the Super PAC that is favorable to Mitt Romney and so he’s making the change.”

The flip shouldn’t affect the race in Ohio too greatly, Sununu said.

“The swing of Mike DeWine from one candidate to another is not a significant thing,” he said. “You love to get endorsements, but the most important thing you can get from an endorser is their organization and, frankly, as I understand it, attorney generals don’t have that much of a political organization.”

Sununu said neither Santorum’s general surge nor DeWine’s defection diminishes the case for Mitt Romney’s electability.

“Nothing has changed in terms of strategy or tactics as we move forward and the fact that the governor is so far ahead in terms of delegates means that so far he’s demonstrated that he’s the most electable,” he said.

I suspect the Santorum campaign will say differently — and rightly so. Santorum continues to poll well nationally and in Ohio: The latest Quinnipiac poll showed that, if the Ohio primary were held yesterday, Santorum would take it with 36 percent of the vote. Mitt Romney polled at 29 percent. Nor is Ohio insignificant; It is winner-take-all, and 66 delegates are up for grabs. No Republican nominee has reached the White House without winning Ohio.

Sununu’s cool spin aside, DeWine’s defection surely stings Romney.

Update: Here’s an excerpt from the official statement from the Santorum campaign:

In October, DeWine endorsed Governor Mitt Romney, but after closer scrutiny of Romney’s record and campaign style, he decided to switch his support to Rick Santorum. DeWine said,

“To be elected President, you have to do more than tear down your opponents,” DeWine said.  “You have to give the American people a reason to vote for you — a reason to hope — a reason to believe that under your leadership, America will be better.  Rick Santorum has done that.  Sadly, Governor Romney has not.”

“For some time now, it has been clear to me that Rick Santorum should be the Republican nominee for President,” DeWine said. “To be frank, I’ve had some sleepless nights.  I could not, in good conscience, be on record endorsing Governor Romney when I knew in my heart that Rick Santorum was the better candidate.”