David Cameron was today urged to distance himself from the "deeply personalised" attacks against Nick Clegg by the No to AV campaign.
Liberal Democrat grandee Lord Ashdown warned that the conduct of the Conservative-backed campaign against electoral reform could be "very damaging" for the coalition.
His comments came amid deepening tensions between the two parties ahead of the Alternative Vote (AV) referendum, which is opposed by the Prime Minister but backed by his Lib Dem deputy.
"This has become a deeply and appallingly personal campaign. It's centred on one personality and that is Nick Clegg," Lord Ashdown, the former Lib Dem leader, told the BBC.
He added that the result of the referendum would not affect the coalition, but added: "The way that this is being fought, the way that our leader, Nick Clegg, is being singled out by a campaign funded by the Conservative Party is, I think, very damaging.
"It must be making Liberal Democrats fighting furious and I am certainly one of those."
He was the second senior Lib Dem in less than a day to heavily criticise Tory campaigning ahead of the May 5 referendum.
"I am frankly shocked that coalition partners can stoop to a level of campaign that we have not seen in this country before," the Energy Secretary Chris Huhne told the BBC's Newsnight programme.
"I think it is damaging. There is no doubt about it.
"I can never remember a campaign that has stooped as low as the No campaign in dredging up stuff that they know is downright lies."
He added: "I think this is the politics of the gutter."
Lord Ashdown challenged Mr Cameron to "disassociate himself from a deeply personalised campaign which, frankly, no British Prime Minister should ever be associated with".
He questioned why the Tories were funding the No campaign "whose principal theme is attacking (the Prime Minister's) chief coalition partner".
And he called on Mr Cameron to explain whether "his comments yesterday about broken promises were a direct attack on Nick Clegg... or if not to please explain what he meant".
"The real issues at stake are far too important to be stifled by the No side's poisonous campaign tactics," he said.
"AV will give voters more power and politicians less, make MPs work harder for your vote and end the scandal of having a system where most people have an MP they voted against.
"It's not surprising that the No campaign can't defend the rotten status quo, but stooping to peddling lies and misleading the electorate will not be tolerated.
"That's why the Prime Minister must intervene in the No campaign, which is at this stage nothing more than a Tory front, and ensure he doesn't deny the British people the open and decent debate which they deserve on this important issue."
Although there is still little evidence of the campaign firing the public imagination, a poll last night suggested that those opposed to the change were faring better and had opened up a 16-point lead.
ICM research for the Guardian found that among those saying they had made up their mind and were likely to cast a ballot, 58% were planning to vote No compared to 42% Yes.
Three-quarters of Conservatives are planning to vote against, as are a small majority of Labour supporters.
Only Lib Dem voters are firmly in favour, with more than two-thirds saying they will vote for the change.
However, the Yes camp can take some solace in the 23% who say they still do not know how they will vote.