A forceful denunciation of the state of relations between the Liberal Democrats and Labour, written by former advisers closely linked to the leadership of both parties, argues that voters are ahead of politicians in wanting to see a "more adult relationship" between political parties.
Opponents of co-operation are warned that tactical voting delivered both parties significant numbers of seats in the 1997 general election, and that - if the tactical vote does not materialise next time - the Labour majority would plummet to less than 60 even if the Conservatives fail to win a single extra vote.
The broadside, in a New Statesman article by former Gordon Brown adviser and leading Blairite Neal Lawson, and Neil Sherlock, an aide to ex-Lib Dem leader Paddy Ashdown, will infuriate the significant grouping of Labour MPs opposed to the Lib-Lab "project".
The level of opposition among many party traditionalists will be clear at Labour's autumn conference, where the party leadership is anticipating a battle over whether to renew the manifesto commitment to a referendum on voting reform for Westminister elections.
The commitment is regarded by the Lib Dems as a minimum demand if cooperation with Labour is to continue.
Calling on their parties to "sink their differences", the two aides urge moves to resume progress towards Mr Blair's professed vision of a "progressive 21st century" dominated by a united centre-left.
They say: "To achieve the prize of a progressive century, we have to give up the winner takes all approach of Westminster."
There is a "yawning gap" between expectations and achievements on issues from health and education to welfare reform, they warn, arguing that the gap can be closed only by the sense of vision created by greater co-operation.
While not calling for merger or a formal pact, they propose greater co-operation to defeat the Tories on issues such as social justice and Europe.
