COLUMBIA, S.C. (AP) - Mitt Romney's Republican rivals are intensifying their efforts to erode if not eliminate his standing as the man most likely to defeat President Barack Obama this fall, often stressing their own prospects over his in the final few days before South Carolina's potentially decisive weekend primary.
The stepped-up challenge to Romney's electability, in paid television advertising, campaign appearances and the first of two pre-primary debates, appears aimed at one of his principal strengths in the Iowa caucuses and New Hampshire primaries.
Voters in both contests said they prized a candidate's ability to beat Obama over a candidate's conservative credentials, a preference that helped the former Massachusetts governor to his pair of victories.
After spending days challenging Romney's record as a businessman, Newt Gingrich unveiled a television commercial on Tuesday that starts with an announcer saying only the former House speaker can defeat Obama.
The ad makes no mention of Romney, instead showing Gingrich drawing cheers from the audience at Monday's debate in Myrtle Beach when he said, "More people have been put on food stamps by Barack Obama than any president American history."
Gingrich also drew sustained applause from businessmen and businesswomen after a speech late Tuesday that made only passing reference to Romney, and none at all to his other rivals. Instead, he outlined his own proposals for lower taxes, less regulation and expanded domestic energy production.
"I believe I am the only candidate in this race who understands the scale of change necessary to get this country working again," Gingrich said. He predicted that if he wins the state's primary, the nomination would follow, adding, "We will run a campaign of paychecks vs. food stamps and we will beat Obama virtually everywhere in this country."
Former Pennsylvania Sen. Rick Santorum and Texas Gov. Rick Perry are also hoping to change the perception of voters in the first-in-the-South primary state, sometimes by planting seeds of doubt, at other points sketching dismaying bleak prospects for the party if Romney is atop the ticket this fall.
"As Republicans, we cannot fire our nominee in September," Perry said to Romney from across the debate stage on Monday. "We need to know now. So I hope you'll put your tax records out there this week so the people of South Carolina can take a look and decide if, you know, we got a flawed candidate or not."
Santorum, campaigning in Lexington, S.C., on Tuesday, said Romney is a candidate in the mold of Bob Dole and John McCain, GOP nominees who led the party to defeat in 1996 and 2008.
"He's never run as a conservative in a general election," he said.
By contrast, Santorum said he had won a pair of elections to the House from a Democratic district in Pennsylvania, then carried the state in successive Senate campaigns, including in 2000, when George W. Bush lost the state.
The former senator also sought to ease concerns about the double-digit defeat he suffered in 2006.
"I led the ticket even though I lost by a lot," he said, noting that five GOP congressmen lost that year in the state, including two who were under federal investigation.
The increased concentration on electability comes in the wake of polls with Iowa caucus-goers and New Hampshire primary voters, who said the ability to defeat Obama mattered more than a candidate's stands on conservative credentials, moral character or experience. In Iowa, 31 percent of caucus-goers interviewed for The Associated Press and the television networks said a candidate's ability to defeat Obama mattered most.
Romney won the backing of nearly half of them, far outdistancing the rest of the field. Another 25 percent of caucus-goers interviewed said they cared most about a candidate who was a true conservative. Romney drew the support of a mere 1 percent of them.
Among the 24 percent who cited strong moral character, his share was 11 percent.
The findings were roughly similar in New Hampshire, where Romney's margin of victory was larger than the slender eight-vote spread he won over Santorum in Iowa.
Thirty-five percent of New Hampshire voters cited an ability to defeat Obama as the most important factor in deciding which candidate to support, and Romney drew the backing of 63 percent of them.
Among the 13 percent who said they wanted most of all a candidate who was a true conservative, Romney was favored by only 13 percent; among the 22 percent who said strong moral character mattered most, the former Massachusetts governor got 26 percent.
The conclusion, according to a strategist for one of Romney's rivals, is that "a candidate has to show that he can defeat Obama before any of the ideological differences (among Republicans) can come into play. ... Voters don't want to vote for someone they think has no chance against Obama."
The strategist declined to be identified by name, citing internal campaign strategy. In the days since New Hampshire, the change in tone has seemed to accelerate.
The five remaining rivals converge Thursday in Charleston for the final debate before Saturday's primary.
No Republican since 1980 has won the party's presidential nomination without first capturing the South Carolina primary. It is unclear if the emphasis on electability has had any impact on Romney's lead in the polls in South Carolina.
But the Romney campaign has responded to Gingrich in the past 24 hours with a new Web video and a telephone conference call to reporters. Both featured former Sen. Jim Talent of Missouri, who was in the House when Gingrich served as speaker of the House.
"If we nominate him, he's going to be an unreliable candidate like he was an unreliable leader in the 1990s," he said.
Romney, back in the state a Wednesday after a fundraising trip to New York, slapped at Gingrich's claims as well "He'd been in Congress two years when Ronald Reagan came to office," he said during an appearance at Wofford College in Spartanburg.
Gingrich taking credit for jobs created during Reagan's administration "is like Al Gore taking credit for the Internet."
That was a retort to Gingrich's claim that he played a key role in two conservative revolutions, one led by Reagan in 1980, and the other that he led as speaker after Republicans took control of the House in 1994 after 40 years in the minority.