MESA, Ariz. – Primed for a fight, Mitt Romney and Rick Santorum swapped heated accusations about health care, earmarks and federal bailouts Wednesday night in the 20th and possibly final debate of the roller-coaster race for the Republican presidential nomination.

Texas Rep. Ron Paul chimed in, saying with a smile that Santorum was a fake conservative who had voted for programs he now says he wants to repeal.

With pivotal primaries in Arizona and Michigan six days distant, the most animated clash of the evening focused on health care.

Santorum, surging in the presidential race, said Romney had used government funds to “fund a federal takeover of health care in Massachusetts,” a reference to the state law 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 Obama’s landmark federal law that Romney and other Republicans have vowed to repeal.

Santorum was also 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 Gov. 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.”

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.

After a brief lull, the campaign calendar calls for 13 GOP primaries and caucuses between next Tuesday, when Arizona and Michigan have primaries, and March 6, a 10-state Super Tuesday.

In all, 518 Republican National Convention delegates are at stake between Tuesday and March 6, three times the number awarded in the states that have voted since the beginning of the year. It takes 1,144 to win the nomination.

The dynamic of the campaign – Santorum challenging Romney – made their clashes inevitable.

Romney said Santorum voted five times while in Congress to raise the government’s ability to borrow; supported retention of a law that favors construction unions; and supported increased spending for Planned Parenthood. He said federal spending rose 78 percent while the former Pennsylvania senator was in Congress.

Santorum retorted that government spending declined as a percentage of the economy when he was in the Senate, and he noted that when Romney was asked last year whether he would support a then-pending debt-limit increase, “he said yes.”

There was a clash over federal spending earmarks as well, and Gingrich sought to intervene as more of a referee than a debate participant.

He said he supported the earmarks that Romney had sought for the Olympics in Salt Lake City in 2002 – then accused Romney of observing a double standard by running television ads attacking Santorum for having voted for different earmarks.

Santorum’s rise in the race has left Gingrich and Paul on the outside looking for a way in. The former House speaker has yet to recover from a campaign nosedive that began after he won the South Carolina primary on Jan. 21, and he is pinning his hopes on his home state of Georgia to begin a comeback March 6.