Mitt Romney and Rick Santorum, who has emerged as Mr Romney's leading challenger in the Republican presidential race, have clashed over the US federal government's power in a high-stakes debate that might have been the last in the roller-coaster campaign to challenge President Barack Obama.

The debate was held in the south western state of Arizona six days before crucial votes there and in Mr Romney's native state of Michigan.

The industrial state is now a must-win for Mr Romney, who won it when he ran in 2008 and had been expected to win there again.

Now, however, Mr Romney faces a surging Mr Santorum, whose candidacy has rebounded in the two weeks since he won three contests on the same day. Mr Romney, meanwhile, still faces scepticism among conservatives who dislike his shifting stances on key issues.

A victory in Michigan - no matter who claims it - would provide essential momentum in the state-by-state race ahead of the 10 contests held on the same day a week later, the huge battle known as Super Tuesday.

Mr Santorum, a former senator, was the debate's aggressor on federal bailouts - a key issue in Michigan, where the US car industry is based. GM and Chrysler have since recovered after taking massive bailouts, forcing Mr Romney to explain a 2008 editorial provocatively headlined, Let Detroit Go Bankrupt.

While Mr Santorum opposed the car bailouts, he tried to exploit his rival's position by saying that unlike Mr Romney, he took a consistent stand when he also opposed the federal bank bailouts after the economy collapsed.

"With respect to Governor 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," Mr Santorum said. "That to me is not a principled consistent position."

Mr Santorum, though, was called a "fake" conservative by Texas Representative Ron Paul for voting for federal programmes that he now says he wants to repeal.

A new AP-GfK poll found Republicans remain about equally divided on whether they would rather see Mr Romney or Mr Santorum capture the nomination. Former House of Representatives Speaker Newt Gingrich and Mr Paul lagged well behind. The poll found that Mr Obama would defeat any of the four remaining Republican contenders in a hypothetical match-up.

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