Rick Santorum under fire in Arizona debate
Rick Santorum and Mitt Romney have clashed in a heated debate in the US state of Arizona days before two states hold key primary elections.
A resurgent Mr Santorum came under attack from his rivals over healthcare, earmark spending and voting to raise the debt ceiling.
The candidates were appearing together for the last time before Super Tuesday, when 10 states will hold primary votes.
The eventual Republican nominee will challenge Barack Obama in November.
Former Speaker of the House Newt Gingrich and Texas Congressman Ron Paul also appeared alongside the front-runners Mr Santorum and Mr Romney in Mesa, Arizona.
The highly-anticipated debate came as Mr Romney and Mr Santorum, who are locked in a tightening race, battled for momentum going into approaching primaries in Michigan and Arizona.
This was the first debate since Mr Santorum rose to national prominence with a triple win in the primaries and caucuses in Missouri, Minnesota and Colorado earlier this month.
'Bridge to nowhere'
At the outset, Mr Santorum came under fire from rival candidates who challenged the former Pennsylvania senator's record on spending.
Mr Romney blasted his main rival for voting five times to raise the debt ceiling, while Mr Paul called him a "fake".
The first major flashpoint of the debate came as the candidates discussed earmarks, a legislative tool that sets aside federal funds for specific projects.
Mr Romney said to Mr Santorum: "When I was fighting for the Olympics, you were fighting for the 'bridge to nowhere'," referring to a $400m (£255m) bridge project in Alaska that was ultimately abandoned.
Mr Santorum went on the attack over Mr Romney's support of federal bailouts, saying: "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."
But while answering a question on education reform, Mr Santorum admitted he made a mistake in voting for the No Child Left Behind Act of 2001, and would repeal the measure if elected.
The two leading contenders also locked horns over social issues, but dodged giving their views on contraception because, analysts say, it could cause problems for the eventual nominee during the general election.
Instead, Mr Gingrich focused on the president, lambasting his support of "infanticide", and Mr Romney said the president had undermined "religious tolerance" in America.
Immigration became a focal point of the debate in Arizona, a border state with Mexico where Latinos make up about 18% of the voting population, according to the Pew Hispanic Center.
Mr Gingrich advocated for the construction of two separate fences across the US border, and Mr Santorum also called for a tougher approach to illegal immigration.
Mr Romney said he supported tough new immigration legislation passed in Arizona in 2010 aimed at cracking down on illegal immigrants and would drop federal efforts to block the measure.
The candidates also roundly criticised the Obama administration for dealing too timidly with Iran.
Mr Gingrich slammed General Martin Dempsey, chairman of the joint chiefs of staff, for calling Iran a "rational actor".
Mr Romney joined the onslaught, saying President Obama's position on Iran was the greatest failure of his administration.
Mr Santorum and Mr Romney went head-to-head over healthcare, with the former senator saying Mr Romney had tried to "fund a federal takeover of healthcare in Massachusetts" during his time as governor.
But Mr Romney hit back that Mr Santorum supported lawmakers who voted for President Obama's landmark healthcare bill.
"He voted for Obamacare. If you had not supported him, if we had said no to Arlen Specter, we would not have Obamacare," Mr Romney said.
Road to nomination
The long series of TV debates have played an important role in shaping this year's Republican primary season.
Wednesday's debate, the 20th of the campaign, has the potential to propel Mr Romney or Mr Santorum towards victory on Tuesday.
Mr Romney's strong debate performance ahead of Florida's influential primary is said to have contributed to his success in that state, while former Speaker Newt Gingrich's South Carolina showing helped him win there.
The winner of Tuesday's contests in Arizona and Michigan could gain critical momentum days before Super Tuesday on 6 March, correspondents say.
Despite expectations that Mr Romney would secure an easy victory in Michigan, the state where he grew up and where his father was governor, recent polls show Mr Santorum closing in on Mr Romney's lead days ahead of the vote.
In the last round of primary voting, Mr Santorum scooped three victories in a single night in Missouri, Minnesota and Colorado - states that Mr Romney won in 2008.
Correspondents say the stakes are especially high for Mr Romney, and losses on Tuesday - particularly in Michigan - could do serious harm to his campaign.
The eventual nominee must accrue at least 1,144 delegates in order to secure the Republican nomination.
But the tight race has prompted recent speculation that a prolonged primary season could end with no candidate past that figure, forcing Republicans to pick a nominee at their August convention.