Business Secretary Vince Cable has urged Labour supporters to back the Alternative Vote (AV) in next month's referendum to help stop the Conservatives winning general elections.
He called on the "progressive majority" to get behind the reform to end the domination of Tory governments elected under first-past-the-post without the support of 50 per cent of the population.
His remarks are likely to inflame tensions between the Lib Dems and their Conservative coalition partners.
As well as making plain his wish for the Lib Dems and Labour to keep the Tories out of office, Mr Cable also questioned David Cameron's failure to prevent members of his party launching "brutal" attacks against Nick Clegg.
The Deputy Prime Minister will also try to make common cause with "progressive" Labour supporters today as he hits the campaign trail in Norwich ahead of the May 5 referendum on changing the voting system for Westminster elections.
In an interview with The Guardian, Mr Cable said it was time to complete the work of Liberal and Labour MPs who tried to introduce AV in the 1930s.
"Conservative governments have dominated ever since without securing a majority of the popular vote," he said.
"It's time for the progressive majority in the country to rise above this narrow tribalism and support this reform because we need to make sure the progressive majority wins elections in this century and not the Conservatives as they did, by the back door, for two-thirds of the last century."
The Business Secretary said he had been "alarmed" by an anti-AV campaign leaflet, promoted by a Tory councillor from Worcestershire, accusing Mr Clegg of breaking promises when he had "loyally supported coalition policies".
"I find it difficult to reconcile this leaflet attacking coalition economic policy with the fact that Mr Cameron is personally leading the No campaign and the leaflet is produced by a leading Tory donor.
"He may not directly control what his supporters are up to. But he must make it clear that he doesn't condone and will endeavour to stop personal attacks on his deputy for loyally supporting coalition policy.
"To stand by and let this happen is dangerous and puts considerable strain on the coalition. I haven't really reacted to this spat. But that leaflet was absolutely dreadful. It does take it on to a different level."
His views echo those of other senior Lib Dems, including the Energy Secretary Chris Huhne and former leader Lord Ashdown, who feel Mr Clegg has been unfairly targeted by the Tories during the referendum campaign.
Asked about relations between the Tories and Lib Dems after the May 5 referendum, Mr Cable said it would work "in a professional and businesslike way to deliver an agreed agenda", but stressed: "That doesn't mean ideological alignment or an absence of policy disagreement and I am not into lovey dovey stuff either."
Mr Clegg, despite the Tory-Lib Dem alliance in Westminster, will say today that he would "rather side with progressives in all parties than the old establishment" - apparently referring to the Conservative Party.
In comments that risk being thrown back at him by his critics, he will also describe Labour figures who disagree with their leader Ed Miliband and oppose AV, as "the means to conservative ends".
"There is a proud history of progressive politics in the Labour Party, just as there is in the Liberal Democrats," he will say.
"A Yes vote would be a victory for progressive politics."
Mr Clegg will acknowledge he is "not exactly every Labour supporter's favourite politician right now, to put it mildly", but urge them to vote for "a real, progressive change to our democracy" in the referendum.
"There are those in the Labour Party who oppose AV. They are trying to block progress," he will go on.
"The John Prescotts, David Blunketts, John Reids and Michael Martins are in the bizarre situation of fighting a campaign that gets the vast majority of its funding from big Tory donors they have always criticised.
"Labour No campaigners are the means to conservative ends. Look at the company they are keeping. The only political parties supporting the status quo are the Conservative Party, the Communist Party and the BNP.
"The contrast with the cross-party Yes campaign speaks volumes. I would rather side with progressives in all parties than the old establishment and the lunatic fringe."