Ed Miliband today urged people not to make the forthcoming referendum on voting reform for Westminster elections a referendum on Deputy Prime Minister Nick Clegg.
Making his first appearance on a "Yes" platform with senior Liberal Democrat and Green politicians, the Labour leader said the alternative vote (AV) system would enable "progressive" parties to come together to defeat the Conservatives.
He was joined on the platform at Westminster Central Hall by former Lib Dem leader Charles Kennedy and Lib Dem president Tim Farron as well as the Green Party leader Caroline Lucas.
Mr Miliband has refused to appear alongside Mr Clegg, arguing that the Lib Dem leader is so unpopular he would be a liability for the campaign.
In his speech however, he said it was important that supporters of AV should set aside their party differences in the referendum.
"People have talked about the implications of a 'Yes' vote for David Cameron or a 'No' vote for Nick Clegg. I urge people to look beyond particular individuals, and vote in the national interest," he said.
Mr Farron indicated his irritation with the dispute over whether Labour was prepared to campaign alongside Mr Clegg.
"I think it would be wrong for us to get into silly rows about who should or should not be on a platform," he said.
Mr Kennedy said it was vital that the supporters of electoral reform took the opportunity presented by the referendum on May 5
"This represents the force of political reform," he said. "This is a chance that has got to be seized."
In his speech, Mr Miliband said that despite the current Lib Dem coalition with the Conservatives, he still believed that AV would help progressive centre-left parties to "build bridges not barriers".
"The tragedy for progressive politics in Britain has been that division on the centre and left has handed a united right victory after victory," he said.
"For most of the last 80 years, there has been one Conservative Party but several competing for progressive votes. No wonder the Tories back the current system.
"They know Britain is not a fundamentally Conservative country. But with first-past-the-post, they are more likely to govern whenever progressive forces are divided."
Mr Farron said he was "absolutely sure" that the Conservatives would honour their agreement to legislate for the introduction of AV if there was a Yes vote, even if it was on a very low turn-out.
Meanwhile, Foreign Secretary William Hague was joined by four former holders of the office - including Labour's Margaret Beckett - in warning it would be a "grave error" to abandon a voting system which had been copied by countries around the world.
"Those of us who have represented Britain internationally know that one of the many reasons why we have always punched above our weight is our simple and straightforward voting system, a system that everyone can understand, because it gives one person, one vote," they said in a letter to The Times.
"Democracies all across the world have been founded on the example of our voting system."
Other signatories included Conservative former foreign secretaries Sir Malcolm Rifkind, Lord Hurd of Westwell and Lord Howe of Aberavon. Labour former Foreign Office ministers Keith Vaz, Tony Lloyd and Caroline Flint also signed.