The Liberal Democrats yesterday rebuffed an attempt by Labour to encourage tactical voting by their natural supporters to head off a Conservative victory on May 6.
As Gordon Brown joined the drive to persuade progressive voters to maximise the anti-Tory vote, Nick Clegg dismissed Labour's move as a "naked and obvious attempt at trying to garner votes".
The issue moved to the heart of the election debate after Lord Adonis, the Transport Secretary, used an article in The Independent yesterday to appeal to Liberal Democrat supporters to back Labour in Conservative-Labour marginals to keep David Cameron out of Downing Street.
In an interview with politics.co.uk, Mr Brown backed comments by Lord Mandelson, the Business Secretary, who suggested that people in the South West should back the Liberal Democrats in the seats where they were locked in battle with the Tories.
The Prime Minister said: "I think what he [Lord Mandelson] is saying is: if you don't want a Conservative government, make sure you don't get one. I want everyone to vote Labour and I want everyone to vote for our party and I want our number of seats to be the highest. But if people don't want a Conservative government then they must make sure they don't let the Conservatives in."
Mr Clegg dismissed the claim by Lord Adonis that there were few policy differences between Labour and the Liberal Democrats: "If there's a commonality of interest it is between the two old parties who keep making the same old mistakes. It was the Labour and Conservative parties that ganged up together, against our opposition, in an illegal invasion of Iraq. It's the Labour and Conservative parties who presided over one of the most unfair tax systems around.
"The truth is that the Labour and Conservative parties are two old-fashioned parties who are much more similar than either are prepared to admit."
"I say to anyone who votes, vote with your heart. Don't be told by politicians like Lord Adonis that you're only allowed a choice of two."
Ed Davey, the Liberal Democrats' shadow Foreign Secretary, called on potential Tory voters in the north of England to back the Liberal Democrats to keep Labour out.
He said: "This real dividing line at this election is between the Liberal Democrats and the cosy 'Labservative' consensus. From their support for over-reliance on banking to the illegal war in Iraq, from backing unfair taxes to stopping political reform, Labour and the Conservatives have conspired against the real change Britain needs."
Denis MacShane, a Labour MP, said that the Labour grassroots disliked the "dishonest" local campaigning by the Liberal Democrats.
He told the Transport Secretary in a letter: "In Sheffield, Lib Dems are throwing everything at winning a Labour seat instead of winning Tory seats in the region. I report all this factually as on the ground among Labour MPs the view may be different to the once you advance."