Early exit polls on CNN gave Romney the edge in Mississippi and Santorum the lead in Alabama
Republican presidential candidate Mitt Romney speaks to supporters during a campaign stop at Kirkwood Park March 13, 2012, Missouri.
White House hopefuls were locked in a three-way fight Tuesday as they battled for dominance in key southern states, which could bust open the unruly Republican presidential race.
US media said the race was too close to call as the polls shut in the deeply conservative states of Alabama and Mississippi in the state-by-state contests to choose a Republican candidate to take on President Barack Obama in November.
Front-runner Mitt Romney wants to lock up the dragging nomination process while his top rival, ex-senator Rick Santorum, is seeking to cement his status as the best alternative to the former Massachusetts governor.
But the race has dragged on for weeks, and all eyes were on former House speaker Newt Gingrich with Tuesday's contests seen as must-wins for his flagging campaign.
The flamboyant Gingrich, who in the early days of the campaign was denounced for what candidates called his "grandiose ideas," is fighting to remain viable after winning just two out of the 26 contests held so far. But he faces mounting calls to pull out and allow conservatives to coalesce around Santorum.
Early exit polls on CNN gave Romney the edge in Mississippi and Santorum the lead in Alabama. In both races, Gingrich was being nudged into third place.
In Mississippi, the exit polls said Romney was at 33 percent, Santorum at 31 percent and Gingrich 30 percent.
The picture was reversed in Alabama where Santorum, the former Pennsylvania senator, had 34 percent, multimillionaire businessman Romney 29 percent and Gingrich 28 percent.
Libertarian Congressman Ron Paul was trailing in both states. Hawaii was also holding a caucus on Tuesday to pick its nominee.
Gingrich campaign chief of staff, Patrick Millsaps, cautioned against a rush to judgment, saying on CNN: "I think we have to wait, I think it will be a long night."
Earlier, 45-year-old insurance executive Troy Wolkow, his wife Mary, 46, and his 19-year-old daughter, Nicole, all said they had opted for Gingrich.
"I think he's the best candidate. His knowledge, his grasp of the issues and he's somebody that inspires me. If he becomes president, I see him as the most capable of being a leader," Troy Wolkow said.
Lori Robertson, 47, voting in Birmingham, Alabama, on her way to work as a physical therapist, earlier said she supported Romney, in part because "he has a stronger chance to beat Barack Obama."
Roy Watkins, 65, chief executive at Cebert Pharmaceuticals, said he chose devout Catholic Santorum. "The values that he has represent me," he told AFP.
In a sign that Santorum is perhaps preparing for defeat, he scheduled a Tuesday night rally in Louisiana, moving his campaign to the state which votes next week, a few days after Illinois.
Gingrich, speaking to Fox News earlier Tuesday, dismissed Romney as the candidate of corporate fat cats and touted his plan to bring down the price of gas to $2.50 from around $4 currently.
"Barack Obama is responsible for the high price of gasoline. He has followed a deliberately anti-American energy policy," Gingrich said, tapping into anger about high prices at the pump which are hurting Obama's popularity.
But in pointed remarks, White House spokesman Jay Carney dismissed what he called a partisan campaign to misrepresent Obama's energy policy.
"The president on his watch over the past three years has taken significant actions to increase... domestic oil and gas production," Carney said.
Santorum has also trained his sights on Romney, saying the party needs a standard bearer in the mold of late conservative icon president Ronald Reagan.
Romney for his part has argued that he alone has the heft to beat Obama.
"It's essential that we have in Washington a president who understands how the free economy works... how free people pursuing their own dreams make a stronger nation. I've had that experience," he said at a Tuesday rally.
Romney is already ahead in the all-important delegate count, having about 40 percent of the 1,144 needed to win the sweepstakes and be crowned the party's presidential nominee at its convention in Tampa in August.
But the Santorum campaign accused him of "fuzzy math."
"The reality is simple: the Romney math doesn't add up and he will have a very difficult time ever getting to a majority of the delegates," the Santorum campaign said in a memo, arguing "time is on our side."
And Gingrich aide Millsaps said late Tuesday: "There will be a long path to Tampa. I think you're going to see us there."