Mitt Romney's wins in the Michigan and Arizona primaries restore him as the front-runner in the U.S. Republican presidential nomination race entering next week's crucal multi-state contests, after fending off a strong challenge by Rick Santorum.

Romney, the early favourite to challenge President Barack Obama's re-election bid in November, faltered ahead of Tuesday's double primaries, facing a potentially devastating blow to his presidential ambitions amid an unexpected surge from his underfunded social conservative rival Santorum in a state considered Romney's childhood backyard.

But the former Massachusetts governor, who was born and raised in Michigan, eked out a narrow victory in Michigan after coasting to victory in Arizona earlier in the night, surviving what could be the fight of his political life and regaining campaign momentum ahead of next week's Super Tuesday votes in 10 states.

The spotlight in the bitter, bruising Republican contest in Michigan that featured waves of accusations and negative ads now shifts to its southern neighbour, Ohio, a key swing state that has struggled in America's economic recovery since the 2008 recession.

Polls in Ohio also suggest Santorum is leading there, and Romney will be without the benefit of his family's legacy.

But in his victory speech Tuesday night in the Detroit area, Romney said his win shows he can come back from being behind.

"Just a week ago, the pundits and the pollsters were ready to count us out," Romney told a crowd of supporters in a packed auditorium in the eastern Michigan town of Novi, not far from where he grew up and spent his formative years as the son of a governor and top auto executive.

"We didn't win by a lot, but we won by enough and that's all that counts."

Romney won 41 per cent to Santorum's 38 per cent, with Ron Paul at 12 per cent and Newt Gingrich, who did not actively campaign in the state, trailing far behind at seven per cent.

Romney, who has the backing of Republican power brokers and a massive fundraising advantage over his rivals, has emphasized his record in the private sector in a bid to show he is most equipped to handle the U.S. economic recovery.  


Rick Santorum speaks to supporters at his election night primary Tuesday night in Grand Rapids, Mich., as his daughter Elizabeth, right, looks on. (Eric Gay/Associated Press)

While taking aim at Obama, Romney also blasted Santorum as a Washington insider and economic "lightweight," as shifts in Michigan's polls forced him to spend more time and resources than he expected fending off his main rival's challenge.

Romney has struggled to connect with the party's conservative base in state elections so far. His flagging poll numbers in Michigan before the vote prompted waves of speculation that anything but a victory here could be fatal to his campaign.

At the same time, Santorum, a devout Catholic known for his staunch opposition to abortion and same-sex marriage, saw his support numbers among religious and right-wing Tea Party movement supporters surge, and threatened to turn the already-volatile Republican race on its head. 

But since Michigan awards its delegates based on final results and not winner-take-all, Santorum doesn't walk away from the state empty-handed.

Speaking to a crowd of supporters Tuesday night in Grand Rapids, Santorum said he and his team came to the "backyard of one of our opponents" as unknowns facing the seemingly hopeless task of succeeding against Romney's larger, heavily funded campaign.

"People said we had no chance here, and the people of Michigan looked into the hearts of the candidates and all I can say is, ‘Michigan, I love you back,'" he said.

While Romney's fundraising dwarfs his Republican opponents, his spend-heavy trend in major states doesn't bode well for his campaign if he becomes the nominee and faces off against the financial juggernaut of Obama's re-election organization.

Romney's vocal opposition to the federal government's bailouts of Detroit's automakers didn't help his campaign, although all three of his Republican rivals shared his position.

But many Michiganders, including a number of Republicans, still refer to the federal assistance package as a "rescue" from ruin, and viewed Romney's opposition to it as nothing less than a betrayal from the son of a former titan of the industry.  

Paul, Gingrich make push in smaller states

Meanwhile, Paul and Gingrich had already moved on to campaign in the states that hold primaries and caucuses on Super Tuesday, which offers 419 delegates up for grabs.

Gingrich's campaign has faltered after his surprise South Carolina win vaulted him to the top of the race last month, only to be badly beaten by Romney in Florida and come nowhere near winning a state since. He has been focusing on southern states like his home state of Georgia, and shows no signs of dropping out of the race.

Paul, a libertarian Texas congressman who draws large crowds with his message of limited government, monetary reform and massive spending cuts, said Tuesday night his campaign would continue to target caucus and non-winner-take-all primary states where he can accumulate delegates.

"We're very pleased with our strategy," he said in Virginia. "There's something about this message of liberty ... that's very attractive to young people."

To seal the nomination, a candidate must accumulate the backing of 1,144 delegates to win the party nomination at the Republican National Convention in Tampa in August.

'Incendiary' comments blasted

Earlier in the day, while fielding reporters' questions at his Michigan headquarters, Romney offered an explanation as to why his once heavily favoured campaign struggled to take off in the state.

Without mentioning Santorum by name, Romney said it was "very easy" to excite the Republican base with "incendiary comments."  

"We've seen throughout the race that if you're willing to say really outrageous things, that are accusative and attacking of President Obama, that you're going to jump up in the polls," Romney said. "I'm not willing to light my hair on fire to try and get support."

Romney appeared to be referring indirectly to Santorum's comment over the weekend blasting John F. Kennedy's famous 1960 speech on his belief in the "absolute" separation of church and state.

Santorum said Kennedy's position made him want to "throw up."

"The idea that the church can have no influence or no involvement in the operation of the state is absolutely antithetical to the objectives and vision of our country," Santorum said in a television interview.

On primary day Tuesday, Santorum backed away from his line on the Kennedy speech, saying he regretted how he stated it.

Santorum had previously drawn controversy by attacking Obama's agenda as based on a "phony ideology — not a theology based on the Bible."

The candidate insisted he wasn't doubting Obama's Christian faith, but was merely questioning his "world view" on environmental issues.

Obama touts auto bailout

Obama, until recently, was deeply vulnerable because of the struggling U.S. economy. But the president's poll numbers have improved in tandem with signs that the economy is headed for a rebound.

In a speech in Washington Tuesday, Obama proudly embraced his auto industry bailout, telling a raucous labour audience that assertions by his Republican presidential challengers that union members profited from taxpayer-paid rescue are a "load of you know what."

"More than one million Americans across the country would have lost their jobs in the middle of the worst economic crisis since the Great Depression," he told the cheering crowd. "In communities across the Midwest, it would have been another Great Depression."

With files from The Associated Press