About 150 social conservatives meeting in Texas sided with Santorum over a home-state favorite, Texas Gov. Rick Perry, and former House Speaker Newt Gingrich.
The gathering reflected their dissatisfaction with Romney over abortion rights and other issues and their belief that they needed to unite behind one contender if they had any hope of derailing the former Massachusetts governor after his victories in Iowa and New Hampshire.
Their decision comes a week before the next contest, Jan. 21 in South Carolina, where social conservatives are an influential voting bloc. But it was not immediately clear what effect the backing will have.
One participant at the meeting, Family Research Council president Tony Perkins, said Santorum, a former Pennsylvania senator, ended up with 75 percent of the vote.
South Carolina's Republican voters are some of the nation's most conservative.
In exit polling from the 2008 Republican presidential contest there, 60 percent of primary voters said they were born-again Christians. Romney, whose Mormon faith is not considered a Christian denomination by some, carried just 11 percent of their votes in 2008, fewer than his 15 percent tally overall. Mormons consider themselves Christians.
Conservatives looking to back someone else have a heavy workload in a compressed period of time.
Although Santorum is Romney's closest rival, he's 18 points behind in South Carolina, followed by Gingrich, Texas Rep. Ron Paul, Perry and former Utah Gov. Jon Huntsman, according to the CNN/Time/ORC International poll. Six percent are undecided, the survey found.
Santorum has risen here since his breakthrough near-tie with Romney in the Iowa caucuses on Jan. 3. He has a robust state organization and is making aggressive inroads with evangelical conservatives, like many of those who were at the Country Ham House in Greenville on Saturday.