BIRMINGHAM, Ala. - Republican presidential candidate Newt Gingrich, running third in the race to challenge President Barack Obama, is counting on primaries in the overwhelmingly conservative states of Alabama and Mississippi on Tuesday to keep alive his already slim chance of winning the nomination.

But polls are showing a tight three-way contest in both deep South states, where front-running Mitt Romney, the former Massachusetts governor, is posting a surprisingly strong showing as is former Pennsylvania Sen. Rick Santorum.

Gingrich, the former speaker of the House of Representatives, needs victories in both states to meet his goal of resurrecting a candidacy that surpassed Romney for a few brief weeks earlier this year when he pulled an upset victory in South Carolina. More recently, Gingrich also won Georgia, the state he represented in Congress for two decades.

A strong showing for Santorum on Tuesday would, however, allow him to finally establish himself as the main challenger to Romney.

He predicted Monday that he would win the nomination if the race remains undecided by the time the party holds its nominating convention this summer.

Though Romney holds a commanding lead in convention delegates, Santorum said the race is about to enter a period where he will face fewer disadvantages. To date, Romney has outspent Santorum and had stronger campaign organizations working for him.

"They are not going to nominate a moderate Massachusetts governor who's been outspending his opponent 10-1 and can't win the election outright," Santorum said on NBC's "Today" show. "What chance do we have in a general election if he can't, with an overwhelming money advantage, be able to deliver any kind of knockout blow to other candidates?"

"We're going to be the nominee," Santorum said, adding later, "Gov. Romney will not make it."

Santorum has nudged Gingrich to step aside, arguing that a head-to-head contest between himself and Romney should "occur sooner rather than later." Gingrich scoffed at the suggestion.

Both Santorum and Gingrich are attacking the more centrist Romney from the far right of the political spectrum and have found significant support from the conservative Republican base.

A win for Romney in Alabama could all but guarantee him the nomination.

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 and religious voters.

Romney has amassed his lead in delegates to the party's national nominating convention in August largely because Santorum and Gingrich have split the conservative vote. That enabled Romney to edge out Santorum by a few percentage points in closely fought primaries in his home state of Michigan and the key industrial state of Ohio.

Santorum, who has battled to be Romney's chief conservative foe, burnished his standing with a decisive win in caucuses in Kansas on Saturday. He also carried contests last week in Oklahoma and Tennessee.

On NBC Sunday, Santorum said Gingrich's recent stretch of weak showings suggested he has few options left in the race. Gingrich placed third in Kansas, where Santorum crushed the opposition, and 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 calling for Gingrich to leave the race.

Santorum and Gingrich both campaigned Sunday in Mississippi. Romney had no campaign appearances Sunday, but his campaign and an allied independent group were far outpacing his rivals in spending on television ads in Alabama and Mississippi.

The weekend contests left Romney with 454 delegates in the AP's count, more than all his rivals combined. Santorum had 217, while Gingrich had 107 and Texas Rep. Ron 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 get 1,144 delegates to clinch the Republican presidential nomination.