BIRMINGHAM, Ala. — Republican presidential hopeful Rick Santorum on Sunday nudged rival Newt Gingrich to step aside, arguing a head-to-head contest between himself and Mitt Romney should ‘ ‘occur sooner rather than later.” A defiant Gingrich predicted victories in Tuesday’s primaries in Alabama and Mississippi and called Romney the weakest Republican front-runner in nearly a century .

Santorum and Gingrich were campaigning hard two days before what has become a potentially decisive Southern showdown for the GOP field.

Losing Alabama and Mississippi would effectively spell the end for Gingrich, who has banked his waning prospects on an all-Southern strategy. The former House speaker’s lone primary wins have been in South Carolina and Georgia, a state he represented in Congress for 20 years.

A win for Romney in Alabama, where polling shows a tight contest between Romney, Gingrich and Santorum, could all but bring the GOP nominating contest to a close. The former Massachusetts governor has built a substantial delegate lead against his rivals but has failed so far to win a state in the deep South, home to the Republican Party’s most conservative voters.

Santorum, who has battled to be Romney’s chief conservative foe, burnished his standing with a decisive win in Saturday’s caucuses in Kansas. The former Pennsylvania senator also carried contests last week in Oklahoma and Tennessee, giving him a toehold in the South.

On NBC’s “Meet the Press,” Santorum said Gingrich’s recent stretch of weak showings suggests he has few options left in the race. Gingrich placed third in Kansas and dead last in Wyoming, whose caucuses Romney won easily Saturday.

“The speaker can stay in as long as he wants, but I think the better opportunity to make sure that we nominate a conservative is to give us an opportunity to go head-to-head with Gov. Romney at some point and hopefully that will occur sooner rather than later,” Santorum said, adding he wasn’t directly asking Gingrich to get out.

Santorum attended church in Tupelo, Miss., Sunday morning and had campaign stops scheduled in Meridian and Gulfport later in the day. Gingrich was also campaigning in Mississippi, where he planned to attend Baptist church services in Brandon and headline a rally there.

The Gingrich campaign also deployed a “truth squad” across Mississippi, led by Bob Walker, a lobbyist and former House colleague now chairing Gingrich’s campaign.

On “Fox News Sunday,” Gingrich compared Romney to Leonard Wood, a U.S. Army general from New Hampshire who ran for the Republican presidential nomination in 1920 but lost on the tenth ballot to Warren Harding.

“He’s not a very strong front-runner,” Gingrich said of Romney. “Almost all conservatives are opposed, which is the base of the party. And I think we are likely to see after the last primary in June, we’re likely to see a 60-day conversation about what’s going to happen.” In August Republicans head to their national convention in Tampa, Fla.

Gingrich also took aim at Santorum, saying his support for earmarks and other spending projects while in Congress had alienated voters from the Republican party in 2006. Republicans lost both the House and Senate that year, and Santorum lost re-election to the Senate by an 18-point margin.

“This is somebody who on a number of occasions had Washington change him — he admits it and he says it’s a team sport. You had to go along to get along,” Gingrich said of Santorum.

Romney had no campaign appearances Sunday, but his campaign and an allied independent group were far outpacing his rivals on the air.

The Romney campaign boosted its television ad spending from $165,000 to $233,000 in Alabama. Restore Our Future, a super PAC run by former Romney advisers, was spending $1.42 million on ads in Alabama and $973,000 in Mississippi.

Romney planned to campaign in Alabama Monday.

Gingrich and Santorum also had support on the air by well-heeled outside groups.

The Gingrich campaign was running about $125,000 in ads in Alabama and $83,000 in Mississippi, while the pro-Gingrich super PAC Winning Our Future was spending $413,000 on ads in Alabama and $250,000 in Mississippi.

Santorum had only a nominal presence on the air in both states. But the Red White and Blue fund, his allied super PAC, was spending $275,000 on ads in Alabama and $235,000 in Mississippi.

Wins in the U.S. territories Guam, the Virgin Islands and the Northern Mariana Islands along with Wyoming helped Romney significantly pad his delegate lead in the race. The Associated Press calculates that Romney now has 454 delegates compared with 217 for Santorum, 107 for Gingrich and 47 for Texas Rep. Ron Paul.

A candidate must win 1,144 delegates to clinch the Republican presidential nomination at the national convention.