The former Massachusetts governor also went after Santorum on earmarks, the specialized spending bills directed to a particular state or program.
"You voted for the Bridge to Nowhere," Romney said to Santorum, referring to an infamous bridge proposal in Alaska that would have been built with millions in federal funds. "I would put a ban on earmarks."
Paul went further, calling Santorum a "fake" conservative. Gingrich dismissed the argument over earmarks as "silly" but said his years as House speaker made him best equipped to bring reform to such Washington practices.
Santorum, for his part, said he had differentiated between "good earmarks and bad earmarks" and supported only those that funded defense and other needed projects.
He also noted that Romney had sought earmarks to fund the 2002 Salt Lake City Olympics. And he blamed Romney anew for championing a health care law in Massachusetts that became the prototype for Obama's health care law, which is detested by conservatives.
"It would be a difficult task for someone who had the model for Obamacare — the biggest issue in this race — to be the nominee of our party," Santorum said.
In rebuttal, Romney said Santorum actually bore responsibility for passage of the health care law that Obama won from a Democratic-controlled Congress in 2010, even though he wasn't in office at the time. Romney said that in a primary battle in 2004, Santorum had supported then-Sen. Arlen Specter of Pennsylvania, who later switched parties and voted for the law Obama wanted.
Santorum also took his lumps from the audience, which booed when he said he had voted for the No Child Left Behind education law even though he had opposed it.
"Look, politics is a team sport, folks," he said of the measure backed by Republican President George W. Bush and other GOP lawmakers.
Santorum's rise in the race has left Paul and Gingrich as outsiders looking for a way in.
Paul has yet to win any primaries or caucuses. He is airing an ad in Michigan, though, challenging Santorum's claim of taking a conservative line against federal spending.
Gingrich, the former Georgia congressman, is pinning his hopes for a comeback on that state on March 6. He was campaigning in Washington state on Thursday and Friday.
Fouhy reported from Washington. Associated Press writer Connie Cass in Washington also contributed to this report.
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