Lord Mandelson has accused the Tories of orchestrating a media "smear" campaign against Liberal Democrat leader Nick Clegg.
As the three party leaders prepared to line up before the cameras for their second television debate, Labour's campaign supremo claimed the Conservatives were in a state of "panic" because their own campaign was in disarray.
He said the appearance of a series of hostile front-page stories in Thursday's newspapers attacking Mr Clegg bore the hallmarks of Conservative communications director Andy Coulson, a former editor of the News of the World.
The Tories dismissed his claim that they were behind the articles as "utter nonsense".
Mr Clegg appealed to people not to be frightened off voting Lib Dem in the election on May 6, declaring that any suggestion in the reports of wrongdoing on his part was "seriously out of order".
The Daily Express branded Mr Clegg's immigration policies "crazy", The Sun accused him of "wobbling", while the Daily Mail revived comments he made about Anglo-German relations in 2002 under the headline "Nick Clegg in Nazi slur on Britain".
The Daily Telegraph, meanwhile, reported that regular payments from three Lib Dem donors were paid directly into Mr Clegg's private bank account in the period before he became party leader.
Lord Mandelson said the sort of smear tactics which the Tories had used against Labour in past elections were now being turned on the Lib Dems, as the upsurge in their support threatened the Conservatives' chances of an outright victory.
"I think the coverage is frankly disgusting," he told BBC Radio 4's The World at One.
"The press stories we have seen today are straight out of the Tory Party dirty tricks manual. These things do not happen at the drop of a hat. This is pure Andy Coulson-style News of the World territory turned into political form."