Lord Ashdown: George Osborne's AV attacks 'bizarre'
Former Lib Dem leader Lord Ashdown has accused Chancellor George Osborne of trying to "frighten" voters off changing the voting system.
He attacked Mr Osborne for criticising the funding of the "Yes to AV" campaign and accused the no camp of mudslinging.
The Tories and Lib Dems are on opposite sides of the campaign - ahead of a referendum on 5 May.
David Cameron said both sides should use "reasonable arguments" but said Mr Osborne had done "nothing wrong".
It comes as a survey suggests public opinion has hardened against a switch.
As the 5 May referendum draws nearer, both the "yes" and "no" sides are gearing up their campaigns.
The Conservatives agreed to a referendum on changing the voting system for Westminster elections from first-past-the-post to the Alternative Vote, as part of the coalition deal with the Liberal Democrats.
The deal allows the two parties to campaign on opposing sides.
If AV were to replace the current first-past-the-post system, it would mean instead of just voting for one candidate, voters could rank candidates in order of preference.
These preferences could be used to decide the outcome in places where no candidate wins more than 50%.
The chancellor sparked anger last week when he said it "stinks" that the main backer of the pro-AV camp was the Electoral Reform Society - whose commercial arm Electoral Reform Services Ltd (ERSL) runs election services.
He claimed that ERSL stood to benefit financially from a switch. The firm has denied the accusation, saying a switch would have "absolutely no impact" on its revenue.
Writing in the Observer, Lord Ashdown said: "Their strategy is clear: throw as much mud as you can, don't let the issue be discussed openly and frighten the public over the next three weeks into voting to preserve the power the present First-Past-the-Post system gives them.
"This strategy stinks of the same odour which has surrounded our politics recently.
"For the chancellor of the exchequer... to claim that there is something 'dodgy' about the Electoral Reform Society donating cash to a campaign in favour of electoral reform is bizarre.
"Worse, for him to casually toss put slurs against a British company - Electoral Reform Services - which has widespread international respect for its impartiality in the conduct of elections and which even the Conservative party get to run their elections, is desperate.
"He graphically shows why we need to change our politics. Why we need to clean it up."
He also criticised another Tory cabinet minister - Baroness Warsi - for suggesting AV would benefit extremists.
But Mr Cameron told Sky News's Murnaghan programme he did not believe the campaign was becoming increasingly bitter.
"We always knew that we would be on different sides of the Alternative Vote referendum debate," he said.
He argued that AV would be "unfair" and was used by "just a handful of countries", while first-past-the-post was tried and tested. But he added: "It should be a reasonable argument on both sides."
He said the most important thing was to explain "why it matters" to a public who were not "exactly fired up" about the referendum.
Mr Cameron said Mr Osborne's point about the Electoral Reform Society was "a fact", adding: "There is nothing wrong with bringing that fact out."
But he added: "We've got to really focus on what that change would mean."
He said whatever the result of the referendum, he believed the coalition government would "go on being a strong and effective government".
In an article for the News of the World, Mr Cameron warned the AV system would be too complex, unfair and costly and warned there would be "no going back" if AV was voted in.
A ComRes survey for the Independent on Sunday and Sunday Mirror - weighted to reflect those certain to vote - found 37% backed AV with 43% against, compared with a 36% to 30% split the other way in January.