MILFORD, Michigan (Reuters) - Republican Mitt Romney assailed presidential rival Rick Santorum on Thursday for abandoning his conservative principles, pushing a line of attack that polls show is helping him close the gap on his rival in the battleground state of Michigan.
Romney fired a new burst of criticism at the former Pennsylvania senator one day after he repeatedly put Santorum on the defensive in an Arizona debate for backing big spending bills in Congress.
"I don't know that I've ever seen a politician explain in so many ways why it was he voted against his principles," Romney told a builders' conference in Phoenix before he headed to Michigan for a campaign swing.
Arizona and Michigan are the next battlegrounds in the state-by-state battle to pick a challenger to President Barack Obama in the November 6 general election, with crucial nominating contests next Tuesday.
Romney lampooned Santorum's comments during Wednesday's debate that he sometimes had to vote for bills he did not like - including spending programs, education reforms and increases in the federal debt - because politics was a "team sport."
"He talked about this as taking one for the team," said Romney, a former Massachusetts governor and venture capitalist. "I wonder which team he was taking it for? My team is the American people, not the insiders in Washington."
While Santorum, a Catholic social conservative, has courted controversy with comments about abortion, birth control and the role of women in the military, Romney seems to have struck a chord with his portrayal of Santorum as a supporter of big spending.
Romney, fighting to regain the top spot in the Republican presidential race, trailed Santorum slightly in two new polls in Michigan on Thursday after lagging his top rival there by as much as double digits in some polls a week ago.
A comeback by Romney would be well-timed, with 10 Super Tuesday primaries scheduled on March 6.
On Thursday, Romney won a restrained endorsement from Michigan's largest newspaper, the Detroit Free Press - with the caveat that he stop "chest-beating" to prove his conservatism and return the focus to his record and collaborative leadership.
The primary in Michigan, where Romney was born and raised and his father was an auto executive and popular governor, has become critical for him.
A loss there would set off alarm bells about Romney's ability to win the allegiance of conservatives and would ensure a long and costly battle to find a challenger for Obama in the general election.
But a win in Michigan and Arizona would put Romney back in command in a race that has seen a series of conservative rivals rise to challenge him only to fall back into the pack.
An American Research Group poll showed Santorum with a 4-point edge on Romney, and a Detroit Free Press/WXYZ television poll gave Santorum a 3-point edge in a poll where nearly half of the respondents said they could still change their minds. Both polls were taken before Wednesday's debate concluded.
Romney's well-financed campaign has turned its sights on Santorum in TV ads, unleashing a barrage reminiscent of its attacks in Florida against Newt Gingrich, a former speaker of the U.S. House of Representatives.
Gingrich rose to the top of the pack after winning the January 21 South Carolina primary but has nosedived in polls after losing in Florida 10 days later.
The Romney campaign, which as of last week had spent about $1 million on two weeks' worth of advertising in Michigan, is running an ad that shows a man in a suit with a briefcase drowning as the narrator talks about America drowning in debt while Santorum supported big-spending projects.
Santorum also is under fire from another Republican hopeful, U.S. Representative Ron Paul, who called him a "fake" fiscal conservative in a new Michigan ad and at the Arizona debate.
Santorum's campaign fired back at Romney with a new ad that reminds conservatives of some of Romney's shifting positions, including his former support for abortion rights and his backing of a Massachusetts healthcare law that became a precursor for Obama's federal overhaul.
"Since Mitt Romney refuses to talk about his own liberal record - we figured we'd show people what Mitt Romney says about Mitt Romney," Santorum spokesman Hogan Gidley said.
The Obama campaign is airing an ad in Michigan taking on all the Republican contenders, accusing them of turning their back on the U.S. auto industry based in the state while Obama was leading a federal bailout that helped turn it around.
A Pew Research Center poll released on Thursday showed a majority of Americans, 56 percent, believe the loans to General Motors and Chrysler were good for the economy, while 38 percent say they were bad.
The debate on Wednesday was Santorum's first time in the political spotlight since charging into a national lead after winning three contests on February 7 in Missouri, Minnesota and Colorado.
The pace of the Republican race quickens dramatically next month, with 22 states holding nominating contests in March, including 10 on "Super Tuesday" March 6.
The broader campaign battleground in March could allow Romney to exercise his financial and organizational edge on his rivals, and put him in a strong position to knock them out and wrap up the race at least by April, if not earlier.
In its endorsement, the Detroit Free Press criticized Romney for shifting away from his past stances and said he was "dead wrong" for opposing the government bailout for the auto industry.
But, the newspaper's editorial board wrote, "Romney, unlike the zealous Rick Santorum, the impulsive Newt Gingrich and the backward-thinking Ron Paul, is preferable to the rest of the field. He is the only one who has the combination of resume and bearing to occupy the Oval Office."
Santorum may have missed his chance to change the campaign dynamic at Wednesday's debate.
"Santorum needed a strong performance in the debate and he didn't get it," Republican strategist Ford O'Connell said. "All eyes were on him. He had an opening and he missed it."