At a campaign rally in Spartanburg, Romney mocked Gingrich's claims to have helped create jobs during his time in Congress, saying "government doesn't create jobs, it's the private sector that creates jobs."
"Congressmen taking responsibility, or taking credit, for creating jobs is like Al Gore taking credit for the Internet," he jibed, echoing a popular -- and mostly erroneous -- conservative attack on the Democratic former vice president.
It refers to a March 1999 interview in which Gore, praised by Internet pioneers for early legislation that helped build the World Wide Web, said that as a US senator he "took the initiative in creating the Internet," a claim roundly mocked on the right as saying he "invented" it.
Romney's campaign unleashed a blistering barrage at the former House speaker, branding him a dangerously erratic leader whose nomination would pave the way for President Barack Obama to win a new term come November 6.
Former senator Jim Talent, who served under speaker Gingrich in the 1990s, accused him of saying "outrageous and destructive" things that forced others to "clean up after him" and helped Democrat Bill Clinton to reelection in 1996.
Former representative Susan Molinari, speaking with Talent on a conference call arranged by the Romney campaign, said Gingrich practiced "leadership by chaos" and warned he would "help elect another Democrat president."
Romney's campaign echoed those themes in a new Internet advertisement and website, both named "Unreliable Leader," that strongly suggested it saw the more conservative former lawmaker as a rising threat ahead of Saturday's primary.
Asked Wednesday about the attacks, Gingrich reportedly said "that's just stupid."
Gingrich trailed the former Massachusetts governor and multi-millionaire venture capitalist -- who won the only two previous nominating contests, in Iowa and New Hampshire -- by double-digits in South Carolina polls.
But a new poll highlighted the frontrunner's potential peril, showing Gingrich closing the gap since New Hampshire and after a bravura debate performance here on Monday.
The CNN/Time/ORC survey found Romney leading Gingrich 33-23 percent among likely voters, just half of his 19-point lead two weeks ago. The poll had an error margin of plus or minus 4.5 percentage points.
Gingrich has worked to position himself as the strongest conservative challenger to Romney, who faces stubborn doubts about his conservative credentials and has yet to rally a majority of Republicans behind him.
At a campaign event Tuesday, Gingrich pressed Christian conservative flag-bearer Santorum -- running third in South Carolina -- and Texas Governor Rick Perry to drop their campaigns and throw their lot with him.
"From the standpoint of the conservative movement, consolidating into a Gingrich candidacy would in fact virtually guarantee a victory on Saturday," he said. "And I'd be delighted if either Perry or Santorum wanted to do that."
Neither candidate appeared poised to heed the call, but time was fast running out for Romney's rivals to deny him the nomination, and much of his fire aimed at Obama.
Speaking at a campaign rally in Rock Hill after Obama blocked construction of a controversial US-Canada pipeline, Romney charged that the Democrat has not "taken advantage" of US energy resources.
"It's been a presidency that's been as anti-investment, anti-growth, and anti-jobs as we've ever seen," he told hundreds of supporters.
Heating up his general election president campaign, Obama is next week to tour five swing states crucial to his reelection chances: Iowa, Arizona, Nevada, Colorado and Michigan after his annual State of the Union address to Congress on Tuesday, the White House announced.
Obama got some mixed news from a new public opinion poll that found nearly one third of respondents said they were worse off financially since he took office in January 2009 promising "hope and change."
American voters are said to "vote their pocketbooks" and public sentiment about how they have fared financially is viewed as a fairly accurate predictor of whether or not they will support an incumbent president.
But a majority of respondents said Republican former president George W. Bush is more responsible than Obama for the nation's current economic woes.
