* 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.