(Image Credit: Chip Somodevilla/Getty Images)

SOUTHFIELD, Mich. - Offering harsh criticism of  Rick Santorum's effort to urge Democrats to vote against him in today's Michigan primary, Mitt Romney distinguished Santorum's actions from his own, insisting that his decision to vote in the Democratic presidential primary in 1992 is not comparable.

"It's very different running for - being a candidate for president, buying ads, and telling Democrats to go, to go mess into a Republican primary and to vote against me," Romney said today at a press conference at his Michigan campaign headquarters. "Particularly when he doesn't describe his own position on the very issues he attacks me for."

Romney, a registered independent at the time, voted for former Massachusetts Sen. Paul Tsongas, who eventually lost the primary bid to Bill Clinton.

"In my case, I was certainly voting against the Democrat who I thought was the - the person I thought would be the worst leader of our nation," said Romney. "In this case, as I recall, it was Bill Clinton. I wanted someone other than Bill Clinton. And certainly and against - I voted against Ted Kennedy, Tip O'Neill, and Bill Clinton. It seemed like a good group to be against."

Addressing volunteers at his Livonia headquarters this morning, Romney called out Santorum for having paid for robo-calls to target Democrats and urge them to vote for Santorum and not for Romney.

"You are making calls to Republicans today, this is a good thing alright," said Romney. "Yeah, yeah, the Santorum campaign is making calls to Democrats today, alright. So we want to make sure to get Republicans out to vote. We want this to be a process where Republicans choose our Republican nominee."

"We don't want the Democrats to choose who they think is the easiest person to run against," he said. "So you guys let's get the calls done and get Republicans out to vote."

Referring to Santorum as an "economic lightweight," Romney questioned the former senator's ability to fix the economy.

"I think what is most undistinguished in his campaign is his lack of understanding of the economy," said Romney. "I think he is an economic lightweight. I don't think he understands the process of job creating. And the reason that I think people will find him hard to elect in a general election if he were to become our nominee is that he doesn't have the very attribute and skill that is most in demand on the part of the American people right now."

Romney's deputy campaign manager Katie Packer Gage offered even harsher criticism of Santorum's robo-call strategy, telling reporters, "For all of Rick Santorum's complaints about negative campaigning, he has launched the most negative phone campaign that we have seen in Michigan in a really long time."

Leaving an event in Kentwood, Mich., this morning, Santorum defended the robo-calls, telling reporters that his campaign did it "to prove the point we can attract voters, we need to win states like Michigan."