Republican Candidates Up Attacks Against Mitt Romney Ahead of South Carolina Primary

PHOTO: Republican presidential candidate, former Massachusetts Gov., Mitt Romney campaigns at Cherokee Trike and More, a motorcycle sales and service business in Greer, S.C. on Jan. 12, 2012.PlayTom Priddy/Spartanburg Herald-Journal/AP Photo
WATCH Jon Huntsman Drops Out of GOP Presidential Race

As the Republican presidential field thins out, the remaining candidates are polishing their line of attack against Mitt Romney in what could be their last opportunity to derail the frontrunner.

Newt Gingrich and Rick Santorum are expected to come out swinging at the former Massachusetts governor tonight at the Fox News/Wall Street Journal Debate. The two focused on their chief rival on the campaign trail today, playing up aspects of his record that aren't likely to sit well with conservatives.

Gingrich, who experts expect to be the most aggressive tonight, hit Romney on not releasing his tax returns.

"We are a conservative party but we don't have to be a stupid party. It does strike me that nominating the guy who lost to the guy who lost, raises the question about your passion for losing and I personally am not in the losing business," the former House speaker said today.

Santorum, meanwhile, criticized Romney for backing ads by his super PAC that have targeted the former senator's earmarks record.

Romney "has a long track record of sending out his henchmen with the super PAC and his own political action committee," he said, to "try to spread disinformation."

"Here we see the frontrunner who is out there with one of the most liberal records in the field, certainly the most liberal record in the field, go out and try to tear down other folks instead of talking about what he wants to do for this country," Santorum said.

For the Republican candidates, South Carolina is, in many ways, a bellwether of how other southern states will vote in their primaries and caucuses.

The winner in the Palmetto State's Republican primary has gone on to win his party's nomination since it became the "first in the South" GOP primary in 1980. Recognizing its significance, the candidates and their super PACs have spent more than $11 million on television ads in the state, buying up virtually every primetime spot.

Unlike Iowa and New Hampshire, South Carolina is a consistent red state. In the 2008 general election, Sen. John McCain won the Palmetto State, even as President Obama won in the other two.

"This is a red-base state that always votes Republican and if Romney can win a Republican base state, it's a tremendous accomplishment for him," Clemson University professor and author David Woodard said.

The former Massachusetts governor is riding high from a wave of back-to-back successes in Iowa and New Hampshire, the first for a Republican candidate since 1976. And some of his rivals' recent attacks have backfired on them.

Romney is also helped by Jon Huntsman, who ended his campaign this morning. Though Huntsman was unlikely to get many votes -- he was trailing comedian Stephen Colbert, who isn't even on the ballot -- even that handful of votes could give Romney an edge, should the results be close.

But the list of challenges facing the former governor is also long. Huntsman's endorsement could actually be more hurtful than helpful to Romney, who is struggling to sell his conservative credentials to South Carolina voters.

"Being branded or embraced by the moderate candidate in this race is probably not something he was looking forward to doing just a couple of days before the vote in South Carolina," ABC News political director Amy Walter said.

The powerful evangelical group has also yet to back him wholeheartedly.

Santorum, Gingrich and Rick Perry are all counting on evangelicals in South Carolina to boost their candidacies. The ongoing issue is that the religious right has yet to coalesce behind one candidate.

But South Carolina is different. In this staunchly conservative southern state, grassroots momentum goes a long way, as does personal courtship. Candidates and their surrogates are well aware of that reality.

Romney's campaign has been particularly aggressive when it comes to personal outreach. Woodard, a Republican, says he receives two to three calls and about three mailers a day, and that's just from the Romney camp alone.

Several other Republicans, he says, have received messages with personal greetings, which shows that Romney is going out of his way to court voters. The former governor has a difficult task ahead of him, because he is viewed by conservatives with skepticism and continues to be questioned about his views on abortion and health care.

Though he is the frontrunner, Romney was notably absent from a gathering of tea party activists this weekend, which Santorum and Gingrich attended.

Romney also reportedly declined an invitation to an anti-abortion event in South Carolina, the only GOP candidate to do so.

"To win, he's got to create some enthusiasm in the base and he's just not conservative enough to incite the base," Woodard said. "But a victory here says, 'Hey, we got a candidate who can reach across the spectrum here.'"

ABC News' Elicia Dover and Shushannah Walshe contributed to this report.