Primary voting starts in Mississippi
Birmingham - US voters in the southern states of Mississippi and Alabama started casting their ballots early on Tuesday for the Republican candidate to challenge President Barack Obama in November.
Frontrunner Mitt Romney hopes to edge closer to locking up his expected nomination while his top rival, ex-senator Rick Santorum, seeks to cement his status as the most viable alternative to the former Massachusetts governor.
Meanwhile former House speaker Newt Gingrich faces what could be a make-or-break test to sweep the conservative southern states and reboot his foundering bid to become the 2012 Republican presidential nominee.
The first contender to reach the magical number of 1 144 delegates will win the Republican sweepstakes and be crowned the presidential nominee at the party's convention to be held at the end of August.
Romney boasts a better than two-to-one lead over second place Santorum in the delegate count, but has been greeted with some scepticism among Bible Belt Southerners who suspect he is not authentically conservative and are skittish about his Mormon beliefs.
The fourth Republican contender, House lawmaker Ron Paul, has a devoted following, but his libertarian message is not seen as playing well in the South and he is not expected to be a major factor in Tuesday's contests.