A senior Cabinet minister has declared that there are so few major policy differences between Labour and the Liberal Democrats that potential supporters of the third party should vote tactically in order to keep David Cameron out of Downing Street.
In a controversial intervention Lord Adonis raises the sensitive issue of tactical voting amid growing signs that Labour strategists regard a hung parliament as their only realistic hope of clinging to power after the general election.
Writing in The Independent today, the Transport Secretary, a former member of the Social Democratic Party and ex-Liberal Democrat councillor, says: "To avoid a Tory government after 6 May, it is vital to grasp now the fundamental Labour-Lib Dem identity of interest. This can best be served by Labour coming out of the election as strong as possible, able to form a government."
Lord Adonis adds: "In Labour-Tory marginals, a vote for the Lib Dems is a vote which helps the Tories against progressive policies. And in Labour-LibDem marginals every Labour MP returned is a seat in the Commons more likely to put Labour ahead of the Tories and therefore better placed to form a government."
Lord Adonis is silent on what Labour supporters should do in seats where the main battle is between the Tories and Liberal Democrats. The implication is that they should back the Lib Dems but a public call for them not to vote for the Labour candidate would risk a breach of his party's rule book.
His intervention will be seen as an attempt to pave the way for a deal in which Mr Clegg's party would support a minority Labour government in key Commons votes. That is thought more likely than a formal coalition.
Privately, some Labour strategists believe an agreement could be reached even if the Tories emerge with more MPs than Labour. They believe a Lib-Lab pact could be forged if the two parties' combined strength would give them an overall majority and the Tories win fewer than 300 of the 650 Commons seats. However, such a scenario would pose a huge dilemma for Mr Clegg. He has hinted he would consider backing the party which secured the biggest mandate – widely seen as the one with the most MPs. To keep the Tories out when they were the largest single party would be a highly controversial move – and could rebound on the Liberal Democrats at the next election, some of their MPs fear.
The Adonis move will not be welcomed by the Liberal Democrats. Mr Clegg, facing a concerted drive by the Tories to oust several of his 63 MPs in the South-west and South, is determined to seize some seats in Labour's traditional northern heartlands. His allies will regard Labour calls for tactical voting as a "one-way street" designed to hoover up the votes of potential Lib Dem supporters. The Conservatives are likely to use any sign of anti-Tory tactical voting to undermine Mr Clegg's party, warning voters they would be "playing with fire" and that backing the Liberal Democrats could result in, "another five years of Gordon Brown".
However, Lord Adonis insists that it is "nonsense" for the third party to pretend it is equidistant between Labour and the Tories. "The truth is that the Lib Dems, for all their local opportunism, have national policy that is similar to Labour's. The difference is that Labour can implement its programme. The Lib Dems have no realistic chance to implement theirs without a Labour government," he says.
While he admits policy differences on criminal justice and asylum, he insists they are dwarfed by, "our common antipathy to a Tory party which, for all its glossy branding, remains wedded to the 1980s in economic, social and European policy".
Yesterday Mr Clegg appeared to side with Labour in its heated row with the Tories over the Government's decision to raise national insurance contributions (NICs). Branding the Tories' plans "a complete con," he said a Cameron Government would have to raise VAT to balance the books.
A YouGov poll for The Sun puts the Tories on 40 per cent (up three points), Labour 31 per cent (down one point) and Liberal Democrats 18 (down one), which would give Mr Cameron an overall majority of four seats.