"I have to admit, I voted for that," the former senator from Pennsylvania said of President George W. Bush's signature education plan of 2001, which is now out of favor with conservatives.
"It was against the principles I believed in," Santorum said during a televised debate in Mesa, Ariz., noting he had home-schooled his seven children.
"But, you know, when you're part of the team, sometimes you take one for the team, for the leader, and I made a mistake," he said.
Rep. Ron Paul, R-Texas, responded: "He calls this a team sport. He has to go along to get along, and that's the way the team plays, but that's what the problem is with Washington."
Santorum -- who polls indicate is in the lead nationally and even with former Massachusetts Gov. Mitt Romney's boyhood state of Michigan -- was put on the defensive several times during the debate, including by Romney for voting as a senator for a measure that financed Planned Parenthood, which supports abortion rights.
Santorum acknowledged he voted for the measure, but said it was part of "a large appropriation bill that includes a whole host of other things."
"I admit I voted for large appropriations bills, and there were things in there I didn't like,'' Santorum said.
For that, he came under fire again from Paul, who said of Santorum: "He's a fake," adding, "The record is -- so bad."
Voters in Michigan and Arizona go to the polls in primary elections Tuesday, followed by the Washington state primary March 3 and the 10-state Super Tuesday primaries March 6.
Super Tuesday involves contests in Alaska, Georgia, Idaho, Massachusetts, North Dakota, Ohio, Oklahoma, Tennessee, Vermont and Virginia.
Romney played up his status as the only candidate on the stage who had never served in Washington.
When Romney noted he had balanced the Massachusetts budget for four straight years, Santorum said that was required by the state's constitution.
"Don't go around bragging about something you have to do," Santorum told Romney.
"Michael Dukakis balanced the budget for 10 years. Does that make him qualified to be president of the United States? I don't think so," Santorum said, referring to an ex-Massachusetts governor who became the Democratic presidential nominee in 1988 and lost to George H.W. Bush.
When Santorum came under fire from Romney for supporting earmarks as a senator, he countered by noting Romney had sought federal money when he was governor of Massachusetts and when he was in charge of the 2002 Winter Olympics.
Romney didn't apologize for the Olympics money, telling Santorum, "Our Games were successful."
"But while I was fighting to save the Olympics, you were trying to defend the 'Bridge to Nowhere,'" Romney said, referring to a much-criticized project in Alaska that became a symbol of government excess in the 2008 presidential campaign.
Former U.S. House Speaker Newt Gingrich of Georgia, who has been out of the spotlight recently, said Romney was right in his views about the auto industry.
"I think that they would have been much better off to have gone through a managed bankruptcy -- I agree with Gov. Romney," he said.
"I think what would have happened is the (United Auto Workers) would have lost all of their advantages," he said.