* Polls show three-way jumble in high-stakes primaries
* Gingrich battling to keep campaign hopes alive
* Romney tries to pull off surprise win in Deep South (Adds Romney, Gingrich quotes, details)
By John Whitesides
BILOXI, Miss., (Reuters) - Republicans Rick Santorum, Newt Gingrich and Mitt Romney are battling for conservative support in Mississippi and Alabama ahead of presidential primaries that polls showed have turned into tense three-way ties.
The stakes are high for all of the contenders in Tuesday's Deep South contests, with Gingrich fighting to keep his struggling campaign alive and Santorum hoping for a knockout blow that would consolidate conservative opposition to front-runner Mitt Romney.
For Romney, a surprise win in either state would be a landmark breakthrough that would signal his ability to capture conservative support in the party's Deep South strongholds and put him on a path to the nomination.
Public Policy Polling surveys showed a three-way jumble in each state. In Alabama, the three candidates were within 2 points of each other, inside the margin of error. In Mississippi, Gingrich led Romney by 33 percent to 31 percent, with Santorum at 27 percent.
"I do need your help," Romney told supporters who turned out in a rainstorm to hear him in Mobile, Alabama. "This could be an election that comes down to a very small margin."
Romney has opened a big lead over his presidential rivals in collecting delegates to the nominating convention but has not been able to capture the hearts of conservatives who distrust his moderate stances as governor of liberal Massachusetts.
Romney's campaign argues his rivals cannot catch him in the chase for delegates, but Santorum raised the possibility that no one would manage to win the 1,144 delegates to clinch the Republican nomination before the party's convention in late August.
"If this race continues on its current pace, it's going to be very difficult for anyone to get to the number of delegates that is necessary to win the majority at the convention," Santorum told reporters after an energy forum in Biloxi, Mississippi.
He said a long, drawn-out primary race to find a challenger to President Barack Obama in the Nov. 6 election would allow the conservative alternative to Romney to rise.
"If we are successful here, it will have a very positive effect," Santorum said. "People in Mississippi and Alabama want a conservative for sure, they want a conservative nominee."
Romney told CNBC that Republicans would be "signaling our doom" if the nominating fight lasts to the convention.
"We need to select someone to become our nominee, get that person nominated, and get focused on President Obama," he said.
A New York Times/CBS News poll released on Monday showed Obama's public approval rating falling sharply to 41 percent from 50 percent a month ago. The drop comes amid rising gasoline prices, setbacks in Afghanistan and talk of war with Iran.
The poll found that in a hypothetical contest against Romney, Obama had the support of 47 percent to Romney's 44 percent - a statistical dead heat considering the survey's margin of error of 3 percentage points.
Gingrich has vowed to stay in the race all the way to the convention but he will face growing pressure to drop out if he cannot pull out a win in the Deep South primaries on Tuesday.
SPLITTING THE NON-ROMNEY VOTE
Alabama and Mississippi, which together have 90 delegates, have big blocs of conservative and evangelical voters who have moved toward Santorum in recent contests. The states award delegates proportionally.
Santorum beat Gingrich in Oklahoma and Tennessee last week, but Gingrich kept his campaign alive with a victory in his home state of Georgia, which he represented in Congress when he was speaker in the U.S. House of Representatives.
"Clearly the non-Romney vote is split between Gingrich and Santorum and clearly the vast majority of Gingrich backers have Santorum as their second choice," said Quin Hillyer, a senior fellow at the Center for Individual Freedom and a Mobile resident.
"If Gingrich were not in the race, Santorum would win Alabama by 15 points," he said.
At the energy forum, Gingrich and Santorum criticized Obama for being unwilling to open more domestic areas and offshore sites for oil drilling.
"If we exist in a world where there is not peak oil, and we exist in a world where the United States can become the No. 1 producer in the world, then you have a total new array of possible policy strategies," Gingrich said.
He ignored his Republican rivals at the forum but Santorum took shots at both Romney and Gingrich, saying they had fallen for liberal views on climate change.
Santorum criticized Gingrich for an advertisement he did with Democratic former House Speaker Nancy Pelosi where they sat on a couch together in front of the U.S. Capitol calling for action on climate change.
"We want to make sure we have a candidate going up against President Obama who can make the case about energy and our future, who can draw a sharp contrast about what's really at stake," Santorum said.
At a later forum in Birmingham, Alabama, Gingrich said his two top rivals would not be able to beat Obama in November.
"We have to win in a principled way, on a big enough agenda with enough momentum that we can actually change Washington decisively or we are not going to get this country back on the right track," he said. "I think I am the only candidate who can do that."
Romney's campaign and his allied Super PAC have heavily outspent Santorum on the air in both states with negative attack ads, but Santorum said he was getting accustomed to the barrage.
"We're used to being outgunned financially," he told reporters. "We're confident that we're going to do well here."