(MSNBC) — Rick Santorum scored victories in the Mississippi and Alabama primaries on Tuesday, depriving Mitt Romney of a signature win in a conservative stronghold and raising fresh doubts about the viability of Newt Gingrich’s campaign.

The former Pennsylvania senator made his case for being the lone, serious Republican challenger to Romney for the remainder of the primary by besting Gingrich in states the former speaker’s campaign had previously said were essential to its long-term viability.

“We did it again,” Santorum said to wild applause from supporters in Louisiana, and amid projections by NBC News that he would win both Mississippi and Alabama.

Romney had hoped to score a victory in Mississippi, thereby proving his ability to win a state that composes part of the heart of the modern GOP. But he appeared to be heading to a third-place finish in both contests, failing to even surpass Gingrich, a candidate who’s weathered growing calls to exit the race.

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