* Alabama, Mississippi have big evangelical voting blocs
* Romney looks to increase his delegate haul
* Voters ask which Republican stands best chance against Obama
By John Whitesides
TUPELO, Miss., March 11 (Reuters) - Republicans Newt Gingrich and Rick Santorum are in a head-to-head battle for conservative, evangelical voters in Tuesday's Mississippi and Alabama primaries, with Gingrich's political survival riding on the outcome.
Gingrich's struggling presidential bid could be at risk with losses in the two states, which would shatter his southern-based comeback strategy and turn up the pressure for him to step aside and allow Santorum to lead the conservative charge against front-runner Mitt Romney.
Both states have large blocs of conservative and evangelical voters, a vital Republican constituency that has moved toward Santorum in several recent contests. When Santorum won primaries last week in Oklahoma and Tennessee, he led Gingrich among evangelicals by double-digit margins.
"You've got two candidates who are appealing roughly to the same sets of voters, so it's a head-on collision," said Merle Black, an expert on southern politics at Emory University in Georgia. "If Gingrich loses in the Deep South, there is no rationale for the continuation of his campaign."
The two most recent polls in Alabama showed Gingrich, Santorum and Romney in a virtual dead heat at the top. A Mississippi poll gave Romney an 8-point edge as Gingrich and Santorum split the conservative vote.
Yet the former House of Representatives speaker, who represented Georgia in Congress, has a slight home-field advantage in the South. It helped him win Georgia last week, and he cruised to a victory in South Carolina in January with the help of evangelicals.
"I like Newt but I might vote for Santorum if I decide that he can get elected," said Rusty Edwards, a power company worker from Pontotoc, Mississippi. "Santorum is younger and he has a lot of good ideas," said Edwards, who described himself as an evangelical Christian and a "real conservative."
Gingrich's troubled marital history also could be a problem for some evangelical voters in the conservative South. He is in his third marriage. "It's an issue for me," said Carolyn Spears, an undecided retired telephone worker from Quinton, Alabama.
Gingrich vowed on Sunday to push ahead to the Republican convention in Tampa in August even if he loses in the South, where Santorum hopes to land a knockout blow against him.
"I'm committed to going all the way to Tampa," Gingrich said on "Face the Nation" on CBS. "We're going to get a lot of delegates in both Mississippi and Alabama and I think the odds are pretty good that we'll win them."
Gingrich canceled a planned campaign stop in Kansas last week to concentrate on Mississippi and Alabama. His southern focus has been driven in part by limited campaign funds, but it is unclear how much his Georgia roots will matter in other southern states.
"Newt has to win these two states. He's done if he doesn't," said Phillip Stutts, a Republican strategist from Alabama.
ROMNEY BENEFITS
A win by Santorum in either Alabama or Mississippi would prove the former Pennsylvania senator's ability to connect with Republican voters in the party's Deep South strongholds and give him a strong argument that it was time for Gingrich to quit.
"The sooner we can get to a two-person race I think the better chance we have to elect a conservative outright," Santorum told reporters in Tupelo, Mississippi, on Sunday after greeting patrons at a local restaurant.
"Governor Romney's strategy is to keep as many people in (the race) as possible," he said.
Romney has benefited as his two top rivals have split conservative votes. Romney, a shaky front-runner, has not won the hearts of the party's core conservative voters in the race to find a challenger to President Barack Obama in the Nov. 6 general election.
But he could at least score a respectable delegate haul from the two states. Party activists said he has decent organizations in the states, particularly in Mississippi.
On a visit to a Birmingham factory on Friday, Romney was careful to keep expectations low. He simply urged the crowd to go out "and give me as many votes as you can find."
George Cassady, a welder from Locust Fork, Alabama, said he was leaning toward backing Romney because he thought he could resurrect the economy and had the best chance to beat Obama.
"Every time you turn around Newt is stepping in it. I don't think he stands a chance of winning," Cassady said.
Santorum, a staunch social conservative, has put hot-button social issues like abortion, birth control and gay marriage on the agenda in recent weeks. But some voters said they were more interested in other topics.
"For me, it's more about who I feel comfortable with," said Elizabeth Martin, a dermatologist from Hoover, Alabama. She said she was undecided but wanted to hear more about the economy, healthcare reform and foreign policy.
A pro-Santorum Super PAC bought ads in Mississippi and Alabama worth up to $600,000 criticizing Romney and Gingrich. Romney's allied Super PAC has been on the air in both states most of the week with nearly $3 million in ads attacking Santorum.
Neither Alabama nor Mississippi have been vital primary contests in recent Republican presidential races, but they moved up in the calendar and joined together this year in hopes of boosting their clout.
Taken together, the two states have 90 delegates of the 1,144 needed for the nomination. Combined, that would be the fourth biggest state haul after California, New York and Texas.
"It's really playing out like we hoped it would. It's a crucial day for some of the candidates to go forward," said Bill Armistead, chairman of the Alabama Republican Party. (Additional reporting by Victoria Allen in Washington; Editing by Alistair Bell and Jackie Frank)