* Fighting to win Republican primary in Michigan
* Presses economic message in hard-hit state
By Steve Holland
DETROIT, Feb 24 (Reuters) - Mitt Romney on Friday declared himself the Republican presidential candidate with "the only chance" to defeat President Barack Obama as he seized on signs that rival Rick Santorum's grip on Michigan appears to be slipping.
Romney vowed to bring fundamental change to rebuild the U.S. economy with fiscally conservative policies, a message he hopes will help him make a comeback in the hard-hit state where he was born and where Santorum is threatening to score an upset.
Romney told members of the Detroit Economic Club that if elected he would seek lower taxes, deep budget cuts, deficit reduction and entitlement reform which taken together would spur a burst in job growth.
He spoke from a podium on the 30-yard line of the Detroit Lions'Field, with the crowd, mostly men in suits, seated in chairs set up on the artificial turf. It was an odd choice of venue as the huge stadium had an empty feel.
"I'm not promising that every day will be easy, or there won't be sacrifice. But I am promising that every day things will get better," Romney said.
Romney said he has the best chance among Republicans in what he acknowledged would be a difficult battle to topple President Barack Obama, who not only has the advantages of incumbency but has a well-funded re-election effort.
"I not only think I have the best chance - I think I have the only chance," he said.
Michigan and Arizona are the next battlegrounds in the state-by-state fight to pick a challenger to Obama in the Nov. 6 general election. They hold crucial nominating contests next Tuesday and will lay the foundation for the 10 states that vote on "Super Tuesday" March 6.
Michigan's widely watched Mitchell/Rosetta Stone poll showed Romney inching ahead of Santorum with 36 percent support to Santorum's 33 percent. Santorum had recently held a double-digit lead in the state in polls.
While the Romney campaign argues he could survive a loss in Michigan, Romney is desperate to avoid an embarrassment in the state where his father was a popular governor.
In his speech, he emphasized his Michigan roots and love for American-made cars produced by the state's car industry, pointing out he drives aMustang and Chevrolet pick-up truck and wife Ann "drives two Cadillacs, actually."
EDITORIAL PAGE HITS SANTORUM
The Obama campaign is airing an ad in Michigan taking on all the Republican contenders, accusing them of turning their backs on the U.S. auto industry based in the state while Obama was leading a federal bailout that helped turn it around.
A less-than-inspiring debate performance by Santorum in Arizona on Wednesday and a series of controversial statements on contraception, abortion and women serving in the U.S. military could be contributing to his dip in support.
On Friday, The Wall Street Journal editorial page, a conservative bastion, lashed out at Santorum, a former Pennsylvania senator.
One writer predicted that if Santorum wins his party's nomination, "all hell will break loose and a great hopelessness will settle in on Republicans." Another writer said that the socially conservative Santorum "has done little to reassure voters that his personal views will not become policy."
Santorum said last week in Ohio when questioned about his personal views against contraception said it is absurd to think that "I'm going to be the uber-czar that's going to try to impose that on the rest of the country."
In his speech, Romney filled in some details of his economic policies, stressing that he would pursue overhauls of Medicare and Social Security, popular benefits programs for the poor and elderly whose rising costs to taxpayers have been difficult for politicians to take on.
"When it comes to Social Security, we will slowly raise the retirement age. We will slow the growth in benefits for higher-income retirees," he said.
But he avoided mention of his opposition to Obama's bailouts of the auto industry, a theme that could be hurting him in Michigan, whose economy is dependent on vehicles.
Debbie Dingell, wife of Democratic Michigan Representative John Dingell, told reporters Romney should stop emphasizing his opposition to the auto bailouts, as members of the United Auto Workers union protested Romney's visit outside the stadium.
"I think he should stop calling attention to it," she said. "I just think he needs to move on. It's not a strong issue for him. I don't understand why he's bashing labor. It just energizes them."
(Editing by Eric Walsh)