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GRAND RAPIDS, Mich. - Rick Santorum responded to Mitt Romney's accusation Tuesday that he is an "economic lightweight" by saying he may not be a "heavyweight" when it comes to the economy, but it's his rival who is a "lightweight" when it comes to "conservative accomplishments."

"I'm not a heavyweight. I'm not a multimillionaire. I'm someone who has gone out and worked hard and learned my economics from shining shoes … and learning how entrepreneurs and people from the bottom up were able to build businesses," Santorum told reporters at his headquarters here. "So I give the governor credit. He's had a lot more success in the field of finances than I have and to his credit, but I've had success where it counts, which is moving forward conservative ideas and getting them passed and convincing the American public to go along with us, and that's the job I'm running for."

Santorum said he's not running to be the "CEO of a company," but he has been a "leading advocate and a successful advocate for conservative principals."

"Gov. Romney has zero," Santorum said. "He is a lightweight on conservative accomplishments, which happens to be more important than how much success … and how much money you've made in business."

Mitt Romney is a multi-millionaire and made his fortune as a private equity executive, but despite the "shining shoes" comment, Santorum is also a millionaire and served 16 years in the United States Senate.

The former Pennsylvania senator also seemed to walk back comments he made Monday while campaigning in Lansing that high gas prices caused the 2008 recession. He said Monday that Americans were unable to pay their mortgages  because of $4-per-gallon gasoline, not because of unsustainable housing prices or reckless lending  practices.

"Certainly the spike in gas prices had a huge impact on the recession in 2008," he said today, but he admitted that gas prices only "contributed" to the economic downturn.

Santorum joined volunteers at his headquarters here making last minute phone calls to try and get people out to vote before the polls close this evening.

"Looks like everything I've seen, it's a pretty close race, that's why we need people here making phone calls," Santorum said, in between calls to voters.

"When I say their name, that usually tips them off it's not a recorded call," Santorum said.

He had to convince at least one person that it wasn't a prank.

"Robocalls I understand are a very controversial thing these days," he said.

The Romney campaign is hitting Santorum hard for just that: a robo-call the campaign put out on the eve of the Michigan primary urging Democrats to cross over and vote for Santorum. If Romney is  not able to win the state he was born and raised in and where his father was a popular governor, the campaign is already trying to spin it as solely because of electoral mischief. The Romney campaign called the Santorum robo-call both "pathetic" and "desperate."

But Santorum said he's just doing what he should be doing to get voters from across the aisle, saying he "welcome[s] anybody's vote who shares my values and wants to see America go in that direction."

"Ronald Reagan would not have been in the 1976 race if he had not gone out and actively courted Democrats in Texas," Santorum said. "The whole point is to have a conservative message, which I have and go out and get conservatives of all stripes to come and join us. That's how we are going to win Michigan. We can't win Michigan or Pennsylvania unless we are able to get conservatives who don't necessarily vote with us all the time to join us in this race."

Of course not all - and possibly very few - Democrats that will cross over share Santorum's vision. ABC News reported earlier today that one Democratic consultant was releasing a robo-call this afternoon urging Democrats to vote for Santorum to embarrass Romney. The goal is also to prolong the primary process and further weaken the one-time frontrunner and hometown son.

In other states that hold open primaries, like New Hampshire where independents can vote in the Republican primary, Santorum said, Romney was "running around and doing anything to try to encourage Democrats and independents to vote for him."

Registered Democrats could not vote in the Republican primary in New Hampshire, but they can in Michigan.

"When he does it, that's OK … and in so doing trying to appeal as moderate to get them to vote for him," Santorum said, referring to Romney. "I'm appealing to them as a conservative and saying please vote for the conservative that is out there and trying to create jobs in manufacturing; that hopefully will appeal to a lot of blue collar Democrats in this state and I welcome their vote."