Gingrich's decision not to campaign in Michigan so far has allowed Santorum to compete against Romney without also having to fend off a rival for the votes of conservatives.
Paul has yet to win any primaries or caucuses.
He has weighed in against Santorum, though, airing an ad in Michigan that challenges the former senator's claim of taking a conservative line against federal spending. The ad says Santorum voted to raise the debt limit five times, and also supported legislation that created a prescription drug benefit under Medicare.
Associated Press writers Kasie Hunt and Charles Babington contributed to this story
Primed for a fight, Mitt Romney and Rick Santorum traded fiery accusations about health care, spending earmarks and federal bailouts Wednesday night in the 20th and possibly final debate of the roller-coaster race for the Republican presidential nomination.
With pivotal primaries in Arizona and Michigan just six days distant, Romney and Santorum sparred more aggressively than in past debates, sometimes talking over each other's answers.
Texas Rep. Ron Paul chimed in from the side, saying with a smile that Santorum was a fake conservative who had voted for programs that he now says he wants to repeal. Former House Speaker Newt Gingrich acted almost as a referee at times.
The most animated clash of the evening focused on health care.
Santorum, surging in the presidential race, said that Romney had used government funds to "fund a federal takeover of health care in Massachusetts," a reference to the state law that was enacted during Romney's term as governor. The law includes a requirement for individuals to purchase coverage that is similar to the one in President Barack Obama's landmark federal law that Romney and other Republicans have vowed to repeal.
In rebuttal, Romney said Santorum, a former Pennsylvania senator, 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. He said that in a primary battle in 2004, Santorum had supported then-Sen. Arlen Specter, who later switched parties and voted for the law Obama wanted.
"He voted for Obamacare. If you had not supported him, if we had said no to Arlen Specter, we would not have Obamacare," Romney contended.
Santorum was the aggressor on bailouts.
While all four of the Republicans on the debate stage opposed the federal bailout of the auto industry in 2008 and 2009, Santorum said he had voted against other government-funded rescue efforts.
"With respect to Governor Romney that was not the case, he supported the folks on Wall Street and bailed out Wall Street — was all for it — and when it came to the auto workers and the folks in Detroit, he said no. That to me is not a principled consistent position."
The debate had a different look from the 19 that preceded it. Instead of standing behind lecterns, the four presidential rivals sat in chairs lined up side by side.
There was another difference, as well, in the form of polls that underscored the gains that Obama has made in his bid for re-election.
An Associated Press-Gfk poll released Wednesday found that Obama would defeat any of the four remaining Republican contenders in a hypothetical matchup. It also found that the nation is showing more optimism about the state of the economy, the dominant issue in the race.
But for two hours, Romney, Santorum, Paul and Gingrich had a different campaign in mind, their own race for the Republican nomination and the right to oppose Obama in the fall.
After a brief lull, the campaign calendar calls for 13 primaries and caucuses between next Tuesday, when Arizona and Michigan have primaries, and March 6, a 10-state Super Tuesday.
Romney is campaigning confidently in Arizona, so much so that his campaign has not aired any television ads.
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