Mitt Romney pulled back in front in the topsy-turvy race for the Republican presidential nomination on Tuesday with victories in Arizona and Michigan, gaining valuable momentum ahead of next week's pivotal Super Tuesday contest.

But Romney's narrow win in his native Michigan failed to dispel doubts about his ability to rally the party's conservative base for what is expected to be a dogfight with President Barack Obama in November's general election.

"Wow! What a night," an obviously relieved Romney told cheering supporters at his state campaign headquarters in Novi after a glowing introduction from his wife Ann. "We didn't win by a lot but we won by enough and that's all that counts."

Rival Rick Santorum -- who placed a close second in Michigan -- sought to cast the result as a victory for a campaign that had been all but written off until he managed to sweep Minnesota, Colorado and Missouri on February 7.

"A month ago they didn't know who we are but they do now," Santorum told supporters. "We came into the backyard of one of my opponents in a race that everyone said, well, just ignore it, you have really no chance here," Santorum said.

"And the people of Michigan looked into the hearts of the candidates and all I have to say is I love you back."

A staunch Christian conservative who strongly opposes abortion and gay marriage, Santorum advanced in the polls by painting himself as the authentic conservative and his multimillionaire opponent as out of touch.

But Romney put in a strong debate performance when it mattered on Thursday and has used his financial muscle to portray Santorum, a former Pennsylvania senator, as a Washington insider.

"Romney gained momentum with these two wins," said Paul Beck, a political expert at Ohio State University.

"But Super Tuesday will likely yield a mixed picture." The former governor of Massachusetts will likely win that state, along with Virginia where Santorum and former House speaker Newt Gingrich are not on the ballot and possibly Vermont.

But he will have a tough time capturing Georgia, Tennessee, or Oklahoma, Beck said.

"Ohio will be the big prize," Beck told AFP. "Santorum leads here now, but his lead is fragile."

Romney currently leads in pledged delegates after having won the more important states so far.

He also has the best-funded and most organized campaign, which will certainly help in what could be a long slog to the Republican National Convention in August.

All four remaining candidates -- including former House speaker Newt Gingrich and congressman Ron Paul of Texas -- have vowed to stay in the race until the convention, where a result might have to be brokered behind the scenes if no one reaches the magic number of 1,144 delegates.

The negative, gaffe-ridden slug-fest which is providing ample fodder for Obama as he builds his reelection campaign ahead of the November 6 general election.

A pugnacious Obama waded into the race Tuesday, tearing into his Republican opponents for their vehement opposition to his efforts to save the Michigan-based auto industry.

In a barnstorming speech to auto workers gathered in the US capital, Obama accused Romney and his fellow Republicans of being on the wrong side of history and of blatant pandering to conservative voters.

Romney has frequently accused Obama of engineering the bailout of General Motors and Chrysler to help his union supporters, but Obama said Romney's charges of crony capitalism is a 'a load of you-know-what'.

"They're still talking about you as if you're some greedy special interest that needs to be beaten," Obama said without naming names.

"To borrow a line from our old friend Ted Kennedy: what is it about working men and women they find so offensive?"

The Republican opposition to the bailout and focus on divisive social issues "has probably cost them Michigan in the general election," said Michael Traugott, a political expert at the University of Michigan.

"We don't have many auto workers left, but it's symbolic for general attitudes about workers in the state of Michigan," he told AFP.

The attack on unions could also undermine Republicans in rust belt states like Ohio and Pennsylvania which are key battlegrounds in the general election, especially if Obama succeeds in framing Republicans as a party of social extremists who sell out working people while pushing huge tax cuts for the rich.