The move comes in the wake of last week's narrow victory over Labour in the bitterly fought Littleborough and Saddleworth by-election, where the Conservatives were forced into third place.
Paddy Ashdown, the Liberal Democrat leader, will publish a new priorities document in September, listing investment in education, probably repeating a demand for an additional pounds 2bn spending, constitutional reform, and environmental initiatives.
These will form the core of the party's agenda, both for the election and for any subsequent co-operation with Labour.
Senior Liberal Democrats are alarmed that the Opposition has watered down its proposals for reform of the House of Lords, and are playing down prospects of an early referendum on proportional representation for Westminster.
But Labour is buoyed by the progress it made in last week's by-election and argues that, even in areas targeted for years by the Liberal Democrats, Mr Ashdown's vote is very soft. A Labour source said: "It would be plain daft, from a position of such weakness, for Paddy Ashdown to start laying down demands."
Both parties have ruled out a formal pre-election pact but Tony Blair, the Labour leader, said on Friday that the by-election would not "disturb sensible co-operation on issues where there are things in common between us".
In a letter to constituency chairmen released yesterday, Mr Ashdown said: "Nationally we have shown - once again - that the Liberal Democrats are a powerful, independent political force.
"There are now more than 5,000 Liberal Democrat councillors across Britain. In a whole swathe of parliamentary seats across the country, the Liberal Democrats are the clear challengers to the Conservatives - the party that can drive this discredited Government out of office."Reuse content