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.