Published: Feb 23, 2012 5:51 PM EST

MESA, Ariz. (AP) - Launched by a bitter debate, Rick Santorum and Mitt Romney are powering into a crucial stretch of Republican primaries and caucuses, one man badly needing money and the other anxious to win over conservative voters.
    
Romney was turning his focus Thursday to tea partyers in Michigan, his birthplace, where cash-strapped Santorum is waging an unexpectedly strong challenge. Romney's been put on the defensive in the auto-building state over his opposition to the government's bailout of car makers.
    
Romney took a pounding on the auto issue in Wednesday night's debate, and President Barack Obama's re-election campaign piled on Thursday. Obama released a TV ad in Michigan accusing Romney and the other GOP candidates of turning their backs on an industry that supports more than 1 million workers in the state by opposing the bailout.
    
A tea party rally Thursday night in Milford, Mich., will give Romney another chance to explain why he opposed the rescue of GM and Chrysler amid the economic crisis but supported bailouts for banks.
    
Santorum, flourishing in the polls, trails in the money chase and is concentrating on beefing up his campaign treasury in hopes of an upset in Michigan's primary on Tuesday. That would cap a rebound that began two weeks ago when Santorum won caucuses in Minnesota and Colorado and a nonbinding primary in Missouri.
    
The 20th debate of the nomination race offered the GOP hopefuls their final face-to-face outing on a national stage before contests over the next 13 days that may well winnow the four-man field.
    
The debate was staged in Arizona, which also votes Tuesday and where Romney is so confident of victory he hasn't aired any television ads.
    
After the Arizona and Michigan primaries come Washington's caucuses four days later. Then 10 states cast ballots on Super Tuesday, March 6.
    
Polls show Santorum leading the field nationally and in several states. Romney and rivals Ron Paul and Newt Gingrich used the televised debate to challenge Santorum, who repeatedly found himself in the hot seat over his record on spending, home-state projects known as earmarks and support for a federal education law.
    
Romney criticized Santorum for support of spending programs when he represented Pennsylvania in Congress, where he served both in the House and Senate. Romney said Santorum voted five times 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 overall while Santorum 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 if he would support a pending debt-limit increase, "he said yes."
    
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 several years ago for the No Child Left Behind education legislation 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 on the outside looking for a way in.
    
Paul has yet to win any primaries or caucuses. He is airing an ad in Michigan, though, that challenges 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.
    
In all, 518 Republican National Convention delegates are at stake between Feb. 28 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.
    
___
    
Fouhy reported from Washington. Associated Press writer Connie Cass in Washington also contributed to this report.
    
___
    
Follow Fouhy on Twitter: http://www.twitter.com/bfouhy
    
Follow Kasie Hunt: http://www.twitter.com/kasie

(Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)