With most precincts reporting, Rick Santorum is the projected winner of the Republican presidential primaries in Alabama and Mississippi. Newt Gingrich appears to have placed second in both contests with narrow leads over Mitt Romney.

Both states allocate their delegates proportionally, so Mitt Romney will retain his wide lead in the race toward the 1,144 delegates needed to secure the party's nomination. But Santorum's victories are a huge blow to Romney's campaign, which was hoping to put a stop to the narrative that the former governor cannot win in the traditional South. The sweep will likely provide Santorum with a major boost of enthusiausm and financial support.

"I want to say first to the people of Alabama, you made a great difference," Santorum said from Lafayette, La. before the results of Mississippi were clear. "I don't think there was a single poll showing me anywhere close to winning Mississippi. Not one," he added--a true statement, though only a small handful of polls were conducted.

Speaking from Birmingham, Ala., Gingrich sounded his usual defiant note amid speculation that he would drop out of the race after failing to win either state in what is considered his strongest region of support.

"In both states, the conservative candidates got nearly 70 percent of the vote. If you're the frontrunner and you keep coming in third, you're not much of a frontrunner," Gingrich said. "Mitt Romney as the inevitable just collapsed."

Prior to the elections, all four candidates spent time campaigning in the region. Santorum had downplayed his chances of sweeping the night like he did in February in Colorado, Minnesota and Missouri, but said it was important to place well in order to carry on.

"We've got to come in here and do well," Santorum said during a stop in Tuscaloosa on Monday.

Over the past month, money from super PACs supporting the candidates poured in both states. According to a Bloomberg News analysis of marketing data, the groups "supplied 91 percent of the 5,592 campaign ads that aired on broadcast television stations in the two states in the past month," with pro- Romney ads making up "65 percent of all ads."

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In the days leading up to the primaries, polls showed tight races in Mississippi and Alabama. The Real Clear Politics polling average in Alabama put Gingrich in the lead by less than one percentage point over Romney, with Santorum trailing by just three percentage points. In Mississippi, the polling data was similar but much spottier, with Gingrich leading Romney by two percentage points and Santorum by five according to a single Public Policy Polling survey.

Republicans in American Samoa and Hawaii are also voting today, but results from those contests won't be made available for several hours.

Up next is a 52-delegate caucus in Missouri--where Santorum won a so-called "beauty contest" election last month--followed by primaries in Puerto Rico, Illinois and Louisiana over the next few weeks.

This story was last updated at 11:25 p.m. ET.

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