WASHINGTON (AP) — The Republican Party's steadily rightward drift, exemplified by the tea party movement's muscle, keeps hitting a quadrennial paradox that frustrates social conservatives: presidential primaries.

For all its success in congressional races, the GOP's right wing repeatedly has failed to unite behind a "movement conservative" to be the party's White House nominee. It happened in 2008 with John McCain, and in 1996 with Bob Dole.

Now social conservatives fear it's happening again in, of all places, South Carolina, virtually the heartland of the tea party. Former Massachusetts Gov. Mitt Romney is running strong in polls there, threatening to sweep the year's first three GOP contests and all but lock up the nomination in Saturday's primary.

More than 100 evangelical and social conservative leaders convened last week in Texas, hoping to slow Romney's march by backing former Sen. Rick Santorum. But they were far from unanimous, and many party activists feel the effort was too puny and too late.

The loose-knit group's lack of cohesion — underscored Monday when some members announced their strong support for former House Speaker Newt Gingrich — illustrates the hard right's historic difficulty in coalescing early behind one strong contender.

Romney, meanwhile, caught a break Monday. Former Utah Gov. Jon Huntsman, widely seen as competing with Romney for moderate-conservatives' votes, dropped out and endorsed the front-runner.

Romney began the contest as the GOP establishment's favorite, running a steady but unspectacular campaign while rivals on his right soared and crashed. Rep. Michele Bachmann and businessman Herman Cain eventually dropped out. Former Alaska Gov. Sarah Palin never got in. Gingrich and Texas Gov. Rick Perry are hanging on, but they have fallen dramatically from their respective high points of last year.

Santorum, virtually an afterthought in the race until Christmas, may have the best chance of becoming the non-Romney candidate. But he lags far behind Romney in money, organization and experience.

There are several explanations, perhaps none of which will satisfy people who want an unabashed, down-the-line social and fiscal conservative as president.

The most benign explanation is that Republicans are so intent on ousting President Barack Obama that they will settle for a far-from-pure conservative nominee and rally around him this fall. Indeed, GOP polls show Romney's perceived "electability" as one his greatest assets.

Tony Perkins, who attended the Texas gathering as head of the conservative Family Research Council, says social conservatism is "choking on its own success" by attracting so many presidential hopefuls.

"The field is so inviting for socially conservative candidates to get in," Perkins said, "they slice up the vote."

But Dan Schnur, a former campaign and policy adviser for Republicans, says conservative activists keep getting outmaneuvered by the party's more pragmatic and mainstream operatives who know how to run campaigns.

Among national Republicans, "a balance of power has shifted from the establishment to the grassroots," said Schnur, who teaches politics at the University of Southern California. "That said, the thing about establishments is: They are established, and they are organized."

Social conservative crusader Pat Buchanan and flat-tax champion Steve Forbes ran in 1996, but the establishment backed Dole, a longtime Senate leader and an uninspiring campaigner.

In 2008, many on the Republican right disliked McCain, the Arizona senator who championed campaign finance limits and thumbed his nose at other conservative orthodoxies. But he easily passed Baptist minister and former Arkansas Gov. Mike Huckabee, who won the 2008 Iowa caucuses.

George W. Bush ran as a "compassionate conservative" in 2000, although he governed more to the right, especially on military matters.

If any state would reject Romney's moderate style and history of supporting abortion rights and gun control, South Carolina would near the top. The state's congressional delegation includes some of the nation's most prominent tea party advocates: Sen. Jim DeMint and Reps. Trey Gowdy, Tim Scott and Joe Wilson, made famous for shouting at Obama, "You lie!"

Yet Romney appears to be coasting, wooing another tea party favorite, Gov. Nikki Haley, to his side. Haley constantly emphasizes the need to oust Obama. Romney, she tells South Carolina Republicans, is the man to do it.

In Monday's debate in Myrtle Beach, S.C., Romney again focused much of his fire on Obama, doing his best to avoid his rivals' efforts to draw him into tit-for-tat arguments.

Those rivals have practically begged voters to reject Romney, or not "to settle" for a quasi-conservative, as Bachmann often put it.

Santorum says Romney disqualified himself, as governor, by insisting that all Massachusetts residents obtain health insurance. Nominating Romney would amount to political "malpractice," he says, because it would undermine efforts to attack Obama's 2010 health care overhaul.

Gingrich has veered from topic to topic at times, but he too has portrayed himself as an uncompromising conservative.

When a New Hampshire voter asked how he could govern without being willing to raise taxes to help close budget deficits, Gingrich replied: "I'm happy to cooperate. I'm not willing to compromise. Compromise in Washington means sell out."

Some conservative activists see an unhappy scenario playing out again.

South Carolina state Rep. Larry Grooms has withdrawn his support of Perry.

"There are a lot of conservatives who were happy to see him get in, and now who would be happy to see him get out," Grooms told The Associated Press. "When conservatives have split in the past, we end up nominating a moderate, and that's not good for our party."

His plea may be coming too late.