The twin primaries on Tuesday follow a pair of weekend contests in the long, bruising fight to pick an opponent to President Barack Obama this fall. Setting the stage Saturday were Rick Santorum's romp in the Kansas caucuses and front-runner Mitt Romney's win in Wyoming.
Polls show a close race in the two upcoming contests, particularly in Alabama, where Romney, Santorum and Newt Gingrich all added to their TV advertising for the race's closing days.
Gingrich, whose campaign is struggling for survival, can ill afford a loss in either Mississippi or Alabama. Romney, the former governor of Massachusetts, is seeking a Southern breakthrough to show he has the ability to win the support of evangelical voters.
Santorum hopes to knock Gingrich out of the contest and finally emerge as Romney's sole challenger from the right.
Speaking Sunday on NBC's "Meet the Press," Santorum said his grassroots campaign has done very well against the better-funded Romney in one-on-one matchups. Still, he didn't call for Gingrich to quit the race.
"I didn't ask Speaker Gingrich to get in, I'm not going to ask him to get out," he said.
A Gingrich aide has said the former House speaker must win both Southern primaries to justify continuing in the campaign. But Gingrich strongly suggested otherwise on Friday as polls showed a tight contest in Alabama.
"I think there's a fair chance we'll win," he told The Associated Press about the contests in Alabama and Mississippi. "But I just want to set this to rest once and for all. We're going to Tampa," sight of the GOP's national convention this summer.
Romney had no campaign appearances Saturday. He won six of 10 Super Tuesday states earlier in the week.
Final returns in Kansas showed Santorum with 51 percent of the vote, far outpacing Romney, who had 21 percent. Newt Gingrich finished with 14 percent and Ron Paul, the libertarian-leaning Texas congressman, trailed with 13 percent.
Santorum picked up 33 of the state's 40 delegates at stake, cutting slightly into Romney's overwhelming's advantage.
In Wyoming, Romney won seven of the 12 delegates up for grabs, Santorum three, Paul one. One delegate was uncommitted.
The contests in Kansas and Wyoming left Romney with 454 delegates in the AP's count, more than all his rivals combined. Santorum had 217, while Gingrich had 107 and Paul had 47.
Romney's totals included 22 that he picked up in the Virgin Islands, Guam and the Northern Mariana Islands.
A candidate must win 1,144 to clinch the Republican presidential nomination at the national convention in Tampa next August.