The mud is starting to fly in the 2008 presidential race, with Hillary Clinton and Barack Obama trading shots over rumors of a secret scandal while Republican rivals of Mitt Romney have been forced to deny being behind an underhand “push-polling” attack on his Mormon faith.

Just days after increasingly acrimonious exchanges in Thursday’s Democratic debate, agents of Clinton were quoted as saying that they had obtained unspecified “scandalous information” about Obama.

His campaign responded furiously over the weekend, challenging Clinton either to make the information public “or concede the truth that there is none”. Obama, who is standing on a platform of changing Washington’s adversarial culture, accused the Democratic front-runner of using “Swift Boat” tactics of “innuendo and insinuation” similar to those deployed in the 2004 race when John Kerry’s Vietnam war record as a Swift Boat captain was smeared.

“I am prepared to stand up to that kind of politics, whether it’s deployed by candidates in our party, in the other party or by any third party,” he said. “The cause of moving America forward demands that we defeat it.

“She of all people, having complained so often about the politics of personal destruction, should move quickly to either stand by or renounce these tactics.”

But Howard Wolfson, Clinton’s spokesman, protested that she had “no idea” what the claim — reported by the conservative columnist Robert Novak — was all about. He suggested that Mr Obama had shown his naivety by falling for an old right-wing trick: “A Republican-leaning journalist runs a blind item designed to set Democrats against one another. Experienced Democrats see this for what it is. Others get distracted and thrown off their games,” Mr Wolfson said.

The weekend row reflects a growing testiness between the two campaigns. Mr Obama has irritated the Clinton camp by implying in recent days that Bill Clinton was executing a 20-year scheme for him and his wife to win the White House. “I’m not in this race to fulfil some long-held plan or because it was owed to me,” he said last week.

The Clintons have vigorously denied the charge, repeated in a recent book about their marriage. In Thursday’s debate she rounded on another rival, John Edwards, for “throwing mud” and pleaded with Democrats to show unity in the face of the Republicans.

But their Republican opponents also spent the weekend embroiled in claims and counter-claims. Romney has denounced as “un-American” phone calls — made under the guise of polling — to voters in New Hampshire and Iowa, the crucial early states kicking off the nominating process, that highlight his Mormon beliefs and suggest that he had “flip-flopped” on social issues such as abortion. John McCain has denied responsibility for the calls, and a spokeswoman for Rudy Giuliani said that his campaign did “not support or engage in these types of tactics”.

The polling was carried out by Western Wats, a market research company based in Utah, the centre of the Mormon faith. A spokesman for Romney’s campaign denied that it had paid for the telephone calls in an attempt to make the candidate appear a victim of dirty tricks.