Republicans Mitt Romney, Rick Santorum and Newt Gingrich faced off in a pair of high-stakes presidential primaries in Mississippi and Alabama on Tuesday, with polls showing a virtual three-way dead heat in both states.

The contests are critical for all three top contenders in a volatile Republican White House campaign that already has featured numerous surges, collapses and voter mood swings.

Gingrich needs a win to keep his struggling campaign afloat, while Santorum hopes to knock Gingrich out of the race and consolidate conservative opposition to Romney.

A victory for Romney in either state, however, would be a critical breakthrough that could put the front-runner on a path to the nomination by proving his ability to appeal to the party's core conservatives in the Deep South.

Polls found the three contenders within a few percentage points of each other in both states, with Romney showing strength as Gingrich and Santorum split the states' big bloc of very conservative, evangelical voters.

"It's very close. We think we have a good chance to win both states," Gingrich told Fox News on Monday night. "We do have a chance here to break through."

Although Gingrich and Santorum have urged each other to get out of the race, Gingrich indicated in a radio interview on Tuesday that he and the former Pennsylvania senator eventually could form a united front against Romney.

"I wouldn't be surprised once we're through the primaries, if it still looks like it does right now, to see the conservatives come together," Gingrich said on the "Rick & Bubba Show" in Birmingham, Alabama. "There's a certain advantage right now in having both of us tag-team Romney, because neither one of us by ourselves can raise the money to match Romney.

"A majority (of Republican voters) are saying, ‘Not Romney,'" Gingrich said. "The biggest bloc is saying Romney, but it's not a big enough bloc to be a majority. We now are beginning to think he will literally not be able to get the delegates to get the nomination."

Romney has opened a big lead in delegates in the Republican race to pick a challenger to President Barack Obama in the November 6 election, but has been unable to capture the hearts of conservatives who distrust him for some of the moderate stances he took as governor of liberal Massachusetts.

Romney, however, rejected the notion that his inability to put away his conservative rivals would mean none of the candidates would have the 1,144 delegates needed to clinch the nomination before the convention in August.

"Can you imagine anything that would be a bigger gift to Barack Obama than us not having a nominee until the end of August? That is just not going to happen," Romney said Monday on CNBC.

Romney currently has about 454 delegates; Santorum 217 and Gingrich 107. On Tuesday, 50 delegates are at stake in Alabama, 40 in Mississippi. There also will be caucuses in Hawaii, where 17 delegates are at stake. In each of the states, candidates are awarded delegates proportionally based on their vote totals.

All three contenders spent the final day before the elections campaigning in Alabama and Mississippi. Romney held a rally in a driving rainstorm in Mobile, Alabama, while Gingrich and Santorum appeared separately at two candidate forums, one in Biloxi, Mississippi, and one in Birmingham, Alabama.

On Tuesday, Romney ventured to Missouri, where Santorum won a non-binding vote last month. Republican officials there will hold caucuses on Saturday to determine how many of the state's 52 delegates will go to each Republican candidate.

During a rally in St. Louis' Kirkwood Park on an unseasonably warm, sunny day, Romney did not mention Santorum or Gingrich. Instead, he focused on Obama, blaming the president for rising gasoline prices.

Romney was scheduled to appear at an event in Kansas City later Tuesday, then fly to New York for fund-raisers. He planned to campaign on Friday in the U.S. territory of Puerto Rico, which holds its Republican primary on Sunday.

Richard Kovach, who attended the rally wearing a T-shirt with the logo of the St. Louis Cardinals baseball team, said he believed Romney will take away some delegates from Missouri.

"He's not going to go away with a big loss," Kovach said.

'YOUR VOTE REALLY MATTERS'

In the hours before Tuesday's contests in Alabama and Mississippi, Gingrich and Santorum called on conservatives to turn out at the polls, despite the rainy weather.

"The primary tomorrow really matters and your vote really matters," Gingrich said in Birmingham on Monday.

During his radio appearance Tuesday morning, Gingrich had a fan in Ray Scott, who is well known in Alabama as the founder of the Bass Angler Sportsman Society, which calls itself the world's largest fishing organization.

Scott called in to the "Rick & Bubba Show" and described Gingrich as "the most qualified man" in the presidential race.

"I think Newt is an absolute God-blessed gift to us," Scott said. "And if we don't go out and vote for him today, somebody ought to be ashamed of themselves."

In Biloxi, Santorum accused Romney and Gingrich of falling for liberal science on the issue of climate change, and he urged the crowd in Birmingham to "elect a conservative, because that's the one who's going to be most electable" against Obama in November.

Santorum, a staunch social conservative, has gained strength among the deeply conservative and evangelical voters who make up a big chunk of the Republican electorate in the two states.

He won conservative Tennessee and Oklahoma last week, while Gingrich kept his campaign alive with a win in his home state of Georgia.

But Romney, who fares the best against Obama in head-to-head poll match-ups of any Republican, has argued his experience as the head of a private equity firm and running the Salt Lake City Olympics makes him the best choice for resurrecting the ailing economy.

Steve Pittler, a voter from Mountain Brook, Alabama, said he thought Romney had the best chance to beat Obama.

"It's more of an ‘anyone but Obama' attitude, and who's the one who can win against him? I think that's Romney," Pittler said.

The other remaining Republican presidential contender, Texas Congressman Ron Paul, largely skipped the Alabama and Mississippi contests to focus on later states in the nominating process.

Gingrich will hold a rally in Birmingham to watch the results in the two southern primaries on Tuesday, but his rivals already are looking to the next contests.

As Romney campaigned in Missouri, Santorum planned to watch the Alabama and Mississippi results come in with supporters in Louisiana, which will have a March 24 presidential contest.