Mitt Romney pulled back in front in the topsy-turvy Republican White House race with victories in Arizona and Michigan, gaining momentum ahead of next week's pivotal Super Tuesday contest.
But Romney's narrow three-point win in his native Michigan Tuesday reinforced 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.
"Wow! What a night," an obviously relieved Romney told cheering supporters in Novi, Michigan.
"We didn't win by a lot, but we won by enough, and that's all that counts."
Romney's decisive and expected victory in Arizona -- where the former Massachusetts governor won 47 percent of the vote and a 21 point lead over main rival Rick Santorum -- was overshadowed by the hotly contested Michigan race.
Santorum -- who placed a close second in Michigan with 38 percent of the vote -- sought to cast the result as a victory for a campaign that had been all but written off before sweeping Minnesota, Colorado and Missouri on February 7.
"A month ago they didn't know who we are, but they do now," Santorum told supporters in Grand Rapids, Michigan.
"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 devout Catholic who strongly opposes abortion and gay marriage, Santorum advanced in the polls by painting himself as the authentic conservative and his multimillionaire opponent Romney as out of touch with ordinary Americans.
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," Paul Beck, a political expert at Ohio State University, told AFP.
"But Super Tuesday will likely yield a mixed picture," he said, referring to March 6, when simultaneous contests will be held in 10 states, offering a trove of delegates and possibly upending the volatile nomination battle once again.
Romney sidestepped what would have been a "devastating blow" but remains vulnerable to Santorum's challenge from the right, said Charles Franklin, co-founder of pollster.com and a professor at Marquette University Law School.
"Santorum meanwhile will have to campaign under the weight of two losses and increased scrutiny rather than the big boost an upset win in Michigan would have given him," Franklin told AFP.
Romney currently leads in pledged delegates after having won the more important states so far.
He also has the best-funded and most organised 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 is providing ample fodder for Obama, who tore into his Republican opponents on Tuesday for their vehement opposition to his efforts to save the Michigan-based auto industry.
Romney has frequently accused Obama of engineering the bailout of General Motors and Chrysler to help his union supporters, but Obama said charges of crony capitalism are 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 in a barnstorming speech to auto workers gathered in the US capital.
"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 successful 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," 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 November 6 general election, especially if Obama succeeds in framing Republicans as social extremists who sell out workers while pushing for huge tax cuts for the rich.