* Michigan key test for both candidates
* Romney explains opposition to auto bailouts
* Santorum, Romney exchange jabs
By Samuel P. Jacobs
DETROIT, Feb 16 (Reuters) - Republican White House hopeful Rick Santorum defended himself against attacks from wounded front-runner Mitt Romney on Thursday as Santorum's hopes rose of dealing a heavy blow in Michigan to his main rival.
Santorum, a former Pennsylvania senator who has emerged as a major threat to Romney in the race for the 2012 Republican presidential nomination, used a speech in Detroit to push a blue-collar theme in the state where the struggling auto industry has help push up unemployment.
Michigan and Arizona on Feb. 28 hold the next contests in the 2012 race that could offer important momentum ahead of the March 6 Super Tuesday jackpot when 10 states vote.
Romney leads in Arizona, but he is running neck and neck with Santorum in Michigan, where Romney's opposition to the 2009 auto bailout is contributing to his risk of losing the state where he was born and where his father was a popular governor.
Romney may get some help from Thursday's endorsement by Michigan's popular governor Rick Snyder.
Nationally, a series of opinion polls this week have shown former underdog Santorum ahead of Romney.
Santorum rejected attacks from the Romney that he was known in Washington as a big spender of taxpayer dollars, insisting he was a fiscal conservative while representing Pennsylvania.
The Romney campaign has used Santorum's support for millions of dollars in government spending projects, including a $400 million "bridge to nowhere" in Alaska, to try to undermine Santorum with conservatives worried about rising debt.
"I was the most conservative senator by far based on the state I represented and the spending record I had," said Santorum, the object of attack ads from Romney supporters.
A day after releasing his tax returns, Santorum took a dig at multi-millionaire former private equity executive Romney.
"I do my own taxes. Heck, Romney paid half the taxes I did. He doesn't do his own taxes. Maybe I should hire an accountant in the future," Santorum said.
Santorum, a strong social conservative, was speaking to the Detroit Economic Club, the kind of pro-business audience more likely to be friendly to his rival.
But Romney has drawn fire here for his criticism of government bailouts of auto giants General Motors and Chrysler.
Romney said Chrysler and GM, which reported a profit of $7.6 billion in 2011, would be even stronger if the government had stayed out of the way. He said the companies eventually went through a "managed bankruptcy process" that allowed them to survive.
"I love the auto industry. I want to see it thriving and growing. I'm glad it went through a managed bankruptcy process, which I recommended from the very beginning to shed unnecessary costs and get its footing again. I'm delighted it's profitable," Romney said in Farmington Hills.
Many of Romney's troubles stem from suspicion among conservatives that he is not one of them. The topsy-turvy 2012 campaign has been marked by the rise and fall of a series of anti-Romney conservative alternatives including Michele Bachmann, Rick Perry, Herman Cain and Newt Gingrich.
Santorum is the most recent and his late emergence gives him a chance of knocking out Romney, who has run a front-runner-style campaign, focusing most of his attention on Obama in hopes of going on to face him in the Nov. 6 presidential election.
Santorum took several shots at Romney, mainly over his vast wealth and his recent comment that he was not worried about the poorest Americans because a safety net already exists for them.
Santorum noted he paid a higher tax rate than Romney, whose effective tax rate was 13.9 percent because his income is based on investments.
Santorum's tax documents showed he built a sizeable income that came partly from lucrative jobs through his Washington contacts. He paid an average of 27 percent in taxes over four years, owns a German-built Audi luxury car and in 2007 claimed a $4,000 deduction for clothing donated to charity.
Of Romney's much-criticized comment about the poor, Santorum said: "He cared about the 95 percent. How about a candidate who cares 100 percent? Who cares about everybody?"
Romney won the endorsement of Michigan's popular governor Rick Snyder on Thursday.
Additional reporting by Sam Youngman; Writing by Steve Holland; Editing by Vicki Allen