US Republican candidates unleashed a new barrage of accusations ahead of South Carolina's key vote, as frontrunner Mitt Romney struggled to beat back his conservative rivals.

The candidates have also faced heightened scrutiny, with US media reporting on the multi-millionaire Romney's offshore accounts and a TV network promising an "explosive" interview with an ex-wife of former House speaker Newt Gingrich.

Gingrich has emerged as Romney's biggest threat after a feisty debate performance Monday, as the state-by-state battle to win the chance to challenge Democratic President Barack Obama in November elections nears a showdown.

With victories in Iowa and New Hampshire already in his pocket, Romney is hoping a win in South Carolina on Saturday will put him on an unstoppable path to being crowned the Republicans' presidential candidate.

But his double-digit lead in opinion polls here appears to be slipping.

A CNN/Time/ORC survey poll on the eve of Thursday's final debate before the primary found Romney leading Gingrich 33-23 percent among likely voters in the state, just half of his 19-point lead two weeks ago.

But ABC News confirmed late Wednesday that it would soon air an interview with the former House speaker's second ex-wife Marianne Gingrich in which she makes "explosive" comments that could harm his campaign.

Gingrich was married when he met Marianne and -- while the couple were still together -- started an affair with his third and current wife, Callista.

Gingrich has expressed regret over his past, but it could harm him among South Carolina's conservative Christians, allowing former Pennsylvania senator Rick Santorum and Texas Governor Rick Perry to split the key demographic.

Supporters of Romney, a former governor of left-leaning Massachusetts, meanwhile branded Gingrich a dangerously erratic leader whose nomination would pave the way for Obama to win a new term come November 6.

Former senator Jim Talent, who served in Congress with speaker Gingrich in the 1990s, accused him of saying "outrageous and destructive" things that helped Democrat Bill Clinton to reelection in 1996.

Gingrich practiced "leadership by chaos" and would "help elect another Democrat president," added ex-congresswoman Susan Molinari.

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 vote.

But Gingrich hit back, defending both his Congress record and arguing that conservatives now were beginning to see him "as the only realistic chance to stop a Massachusetts moderate."

"I think by Saturday we'll be ahead, and depending on how many conservatives come home we could be ahead by a pretty comfortable margin," he told CNN.

He added that under his leadership there were "four consecutive balanced budgets, $405 billion in debt paid off ... unemployment dropped to 4.2 percent. I'd say that's pretty darned good leadership style."

Gingrich has worked to position himself as the strongest conservative challenger to Romney, who faces stubborn doubts about his credentials and has yet to rally a majority of Republicans behind him.

Romney, who is also dogged with the perception that he is out of touch with Americans, faced a potentially dangerous new front as ABC News reported he had millions of dollars in notorious tax havens like the Cayman Islands.

Campaign spokeswoman Andrea Saul confirmed he had money there but said the funds were "taxed in the very same way they would be" on US soil.

Romney meanwhile hit out at Obama at a Wednesday rally in Rock Hill after the president blocked construction of a controversial US-Canada pipeline, charging that the president 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.

In an interview with Time magazine, Obama rejected criticism from Republicans over foreign policy, particularly his handling of Iran's nuclear ambitions -- saying they were "playing to their base."

Next week Obama will 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 said.