Santorum wins Louisiana GOP primary
Romney, a former Massachusetts governor, was projected to finish second. Former House of Representatives Speaker Newt Gingrich was third, but immediately rejected suggestions he should drop out. Texas congressman Ron Paul ran fourth.
Santorum had strong support in all income groups except for the very wealthy, and solid backing from conservative and religious voters in the Deep South state.
"The people of Louisiana sent a loud and clear message - this race is far from over," Santorum said in Green Bay, Wisconsin, where he watched the election results after making a series of campaign stops.
Santorum, a socially conservative former U.S. senator from Pennsylvania, has done well in the Deep South, winning Mississippi and Alabama earlier this month. In all, Santorum has won seven state contests in March.
Saturday's primary allocated only 20 of Louisiana's 46 total Republican delegates as candidates spar for the right to take on Democratic President Barack Obama in the November general election. The rest of the delegates will be allocated at the state party convention in June.
With 91 percent of the precincts reporting, Santorum had 49 percent of the vote, Romney 26 percent and Gingrich 16 percent. If those totals hold, Santorum and Romney would both gain delegates from Louisiana, with Gingrich shut out.
Exit polls showed that 47 percent of Republican primary voters identified themselves as "very conservative," and of those Santorum beat Romney by 52 percent to 24 percent.
Santorum also won majorities among voters calling themselves "somewhat conservative" or "moderate," and those who said that religion matters a lot in public life.
A Republican candidate needs 1,144 delegates to secure the party's nomination. CNN's current delegate count puts Romney well ahead, with 563 delegates to Santorum's 259.
Voter turnout low
Exit polls showed voters in Louisiana, which has a relatively low 6.9 percent unemployment rate but also low median income, were most concerned about the economy and the budget deficit.
The state has been hammered economically in the past decade, first by Hurricane Katrina in 2005 and then by the 2010 Deepwater Horizon oil spill in the Gulf of Mexico.
Still, Santorum found backing from the state's many Christian evangelicals. Louisiana also has a large block of Roman Catholic voters who, as a group, are regarded as more conservative than Catholics in many other states.
"He's a Christian man. I like many of the things he says, about gasoline prices, and healthcare. He's for everything I'm for," said Ronald Moore, 78, a retired commercial contractor from Monroe.
Voter turnout at many precincts was low, according to election officials, even though state Republican leaders had hoped that having the nomination still in the balance would boost turnout.
All four remaining Republican candidates visited Louisiana in the days leading up to the vote.
Gingrich spent a week here, crisscrossing the state from Shreveport in the northwest to Port Fourchon on the Gulf of Mexico, pushing a plan for increased energy production that he said could lower U.S. gasoline prices.
Despite his poor performance, Gingrich vowed to stay in the race as he attempts to play the role of spoiler to Romney's presidential ambitions.
"This is clearly still an open race," Gingrich said in an emailed statement. "I will carry our solution oriented campaign to (the Republican convention in) Tampa."
The next showdown in the Republican nominating contest will be in Wisconsin on April 3. Maryland and the District of Columbia also vote that day.
Santorum has pledged to stay in the race until the party's convention in August, and is likely to take heart from results in March, a month when he has won seven state contests despite failing to chip away at Romney's delegate lead.
For Romney, Louisiana's results suggest that he continues to struggle with the most conservative element of the Republican Party, some of whom distrust his Mormon religion and history of changing positions on social issues.
"Mitt Romney would be really good at creating jobs, but the pro-life issue is one of my main things," said Cheryl Stephens, 56, of Monroe.
Exit polls showed that a now-infamous comment this week by a Romney aide, that the election could be reset like an Etch-A-Sketch after the primaries, might have given Santorum a late lift. Santorum and Gingrich made frequent campaign appearances brandishing the popular toy, and chiding Romney for what they said were his inconsistent positions.
Nineteen percent of voters said the comment was "important" to their vote, and of those, 60 percent went for Santorum.
(Writing by Ros Krasny; Editing by Eric Beech)