Reported by  Shushannah Walshe and Michael Falcone:

DES MOINES, Iowa - Presidential candidate Rick Santorum said he was "taken aback" by Mitt Romney's $10,000 bet at Saturday night's ABC News presidential debate, suggesting that "a nickel or a dollar" would have been a more appropriate amount.

"I was a little taken aback by it," Santorum told reporters on Monday after a campaign event here. "That would not be a number I would have thrown out," he said.

"I either say a nickel or a dollar. I use, 'I'll betcha' a dollar' or 'I betcha nickel.' It's the substantive argument not the money, and as the father of 7 children, nickels and dollars are easier to come by than $10,000," Santorum said.

But the former Pennsylvania senator, who has spent more time campaigning in Iowa than any of his opponents, thanked rival Rick Perry for raising the issue of Romney's past statements that indicate he once thought the Massachusetts health care plan was a "model for the nation."

"It was in his book," Santorum said, referring to Romney's  "No Apology." "I've seen it. I don't know why Mitt keeps running away from it. If you changed your mind - this is not something new - he changed his mind."

Santorum's campaign also sent out a fundraising pitch to supporters on Monday, seeking $10,000.

"Mitt Romney is a multimillionaire former venture capitalist, so we understand that $10,000 might not be a lot of money to him. But it is to us and we are sure it is to most of you," the email message read. "That's why we are betting on our grassroots supporters to help us raise $10,000 today."

After his campaign event, held at the downtown Des Moines offices of the Principal Financial Group, Santorum also turned his fire on Newt Gingrich. He told reporters that he was troubled by Gingrich's past work for Freddie Mac.

"Look, I think the fact that he went out and lobbied for an organization, in my opinion, was not consistent with the conservative values we have," Santorum said. "I just wouldn't do that. I worked for a company when I got out of Congress."

Santorum later clarified that Gingrich wasn't actually registered as a lobbyist but was "someone who spoke on behalf of Freddie," and that, he said, would be enough to create a liability for the former House speaker should he win the Republican nomination.

"There is a difference between someone who goes out and speaks on behalf of the company and someone who is actively engaged in meeting with members of Congress, but it's a difference - it's not apple and oranges," he said. "It's different kinds of apples and that's what I think the American public may see and certainly I can assure you that's what Barack Obama is going to see if he's the nominee of the party."

Members of Santorum's staff also passed out a copy of a May 5, 2006, letter signed by Santorum and other members of Congress warning of the "enormous risk that Fannie Mae and Freddie Mac pose to the housing market, the overall financial system, and the economy as a whole."

Another opponent trying to win the state, Michele Bachmann, announced that she was going on a 10-day marathon tour of all 99 Iowa counties - a feat Santorum completed last month. He told reporters he's "glad she's doing it."

"She gets out and talks to the people of Iowa as you know we continue to go out there," he said. " I've been to most of these counties twice now. I'm glad she's shown that I'm a good person to follow."