Romney rattled by Republican rivals Mitt Romney speaks as Newt Gingrich listens at the South Carolina Republican presidential candidate debate in Myrtle Beach, South Carolina. Photograph: Charles Dharapak/Pool/Corbis Republican presidential frontrunner Mitt Romney appeared rattled after being challenged by his rivals in a televised debate over his failure to disclose tax returns and the millions of dollars spent by his supporters on negative ads. The debate on Tuesday night, at times raucous, came only days before the potentially decisive South Carolina primary, possibly the last state in which Romney's main opponents can feasibly stop his run for the Republican nomination. Although Romney enjoys an 8% poll lead over his nearest rival, Newt Gingrich, his uncomfortable performance watched by millions of viewers, many of them from South Carolina, may have cost him votes. The number of candidates in the debate, the 16th so far, has been reduced to five, following the departure of Jon Huntsman earlier in the day. The main task of Gingrich and Santorum was to rein back Romney and both succeeded, leaving him at times looking hesitant, uncertain, irritated and overly defensive. If Gingrich, the clear winner on the night, was to wound Romney again on Thursday night in Charleston, South Carolina, in the final debate before the primary, the outcome may yet be in the balance. The debate was conducted against a noisy background, with 3,000 partisan Republicans in the hall, booing and heckling, saving applause for their favourites, mainly Gingrich and former senator Rick Santorum. Romney, viewed as too moderate in right-leaning South Carolina, appeared to have failed to fill the hall with his supporters. One of the biggest flashpoints was over the use of negative campaigning by super-political action committees groups of supporters supposedly independent of the candidate, often with millions of dollars at their disposal. Romney claimed he had no control over a super-pac that has spent $3.4m (£2.2m) in negative ads aimed at Gingrich. "That's something that's completely out of the control of candidates," Romney insisted. It is technically true, with election law suggesting that a candidate be independent of the super-pac, but not in reality, with the super-pac filled with Romney's ex-staff and former business partners. Gingrich shot back: "It makes you wonder how much control he would have as president." Romney countered that a super-pac supporting Gingrich had put out a short film with inaccuracies about Romney. "You have a super-pac ad that attacks me," Romney said. It's probably the biggest hoax since Bigfoot. Gingrich said his super-pac had approached Romney's campaign offering to correct any mistakes. The debate was dominated by differences over process rather than policy, mainly because four of the candidates are in broad agreement on major issues, with only the fifth, Ron Paul, out on a limb on foreign policy and cutting federal spending. One of the most significant moments came when Romney, pushed by Texas governor Rick Perry, tentatively agreed to release his tax returns. He has repeatedly resisted this but caved in after questioning by Perry. "Mitt, we need for you to release your income tax so that the people in this country can see how you made your money," said Perry. Romney said the tradition was that candidates released such details in April and he would "probably" do the same, adding he had nothing to hide. Gingrich has promised to release his tax returns on Thursday. Barack Obama, rather than Romney's Republican rivals, may be the main beneficiary if the tax returns confirm the extent of Romney's personal wealth. Santorum too turned on Romney over negative ads, saying that one ad suggesting he supported allowing criminals to vote was untrue and challenged Romney to back it up. Santorum did not back down, saying the bill was justified given that so many African-Americans had served time in jail and would be excluded from the democratic process. He cited Romney's record as governor of Massachusetts, which had an even more lenient policy, allowing criminals out on parole to vote. Gingrich, though he has been heavily criticised by fellow conservatives for doing so, returned to criticism of Romney's business record, in particular closing down companies and laying off workers. "There was in a pattern in some companies, a handful of them, of leaving them with enormous debt and then within a year or two or three having them go broke," he said. That is something he ought to answer for. Perry, trailing in the polls, is likely to quit after Saturday's primary. At the weekend, he came out in defence of the US marines in Afghanistan urinating on dead Taliban but rowed back during the debate, saying they should be punished.