Tapping into the state's religious fervor at his Rock Hill rally, Gingrich pledged to fight "anti-Christian bigotry." In TV ads, he chided Romney for being inconsistent in opposing abortion.
Perry pushed his patriotism in a state with a large military presence. He highlighted his service as an Air Force pilot with a new TV ad featuring veterans praising his character.
Ron Paul, who finished second in New Hampshire, made a more unusual appeal to service members and veterans, emphasizing his anti-war message.
He told a cheering crowd of about 300 in West Columbia that the U.S. should bring its soldiers home from war and stop meddling in other countries' affairs. Paul said military personnel are sick of wars that drain the nation's resources and hurt families.
All the candidates were campaigning in the state Wednesday. Perry, who skimped on New Hampshire to focus on South Carolina, had already been there for days.
Rick Santorum, the former senator from Pennsylvania who nearly tied Romney in the Iowa caucuses, assured supporters in Ridgeway that he wasn't ready to drop his challenge. "It's not just going to be here. It's going to be Florida and beyond," he said.
Obama's team, treating Romney as their likely general election opponent, has joined the effort to darken the picture of his days in private enterprise. Vice President Joe Biden said Romney had worried more about investors doing well than he did about the employees of companies bought by his venture capital firm.
Romney offered a practical-minded defense of layoffs that might not reassure voters worried about holding onto their jobs. "Every time we had a reduction in employment it was designed to try and make the business more successful and, ultimately, to grow it," he told ABC's "Good Morning America."
He also turned the issue on Obama, saying that the president's "been a venture capitalist at Solyndra," the solar company that received a $528 million government loan before going bankrupt, sparking outrage and investigations.
TV ads are filling South Carolina's airwaves, including negative spots like the Gingrich one assailing Romney on abortion, an issue that resonates strongly with evangelicals who make up the GOP's base here.
"He governed pro-abortion," the Gingrich ad says. "Massachusetts moderate Mitt Romney: He can't be trusted."
About $3.5 million has been spent on TV ads in South Carolina, the bulk of it by Perry and a supportive super PAC. But that doesn't count the $3.4 million a pro-Gingrich group has pledged to spend to go after Romney, or the $2.3 million a pro-Romney group plans to spend in the coming days.
Santorum and a super PAC friendly to him also are pouring money into the state, as is an outside group working on Huntsman's behalf.
Associated Press writers Shannon McCaffrey, Brian Bakst and Charles Babington in South Carolina and Connie Cass in Washington contributed to this report.
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