In his first big speech since Mr Blair launched his initiative on "stakeholding", Mr Ashdown will warn that "warm words and weak promises will pave the way for a rejuvenated, Portillo-led right four years later".
The Liberal Democrat leader will set out key areas where the opposition parties need to clarify and sharpen policy in order to forge a post-election alliance.
Addressing the Lib Dem News's first annual lecture, Mr Ashdown will not mention pacts or deals, but he will spell out areas where he believes more clarity from Labour to be vital. They include education, the economy, welfare reform, the environment and Europe - policy areas where consensus is vital to achieve permanent reform.
In his speech, which will be delivered at London's Institution of Civil Engineers, Mr Ashdown will argue: "Those who see the winning of the next election as an end in itself, rather than a means to a possible end beyond it, are paving the way for a four-year disaster."
Mr Ashdown's allies say he is concerned that the opportunity to eject the Tories for a lengthy period of time may be squandered. They cite the example of Bill Clinton, the United States President, who came to power with much public goodwill but whose authority was damaged as his administration became buffeted by events.
Mr Ashdown, who will call for constitutional reform including changes to the voting system, will argue that a one-term Labour victory could be counter-productive. He will say: "What we need is not just a tactical victory over the Conservatives but a strategic change in the way this country is governed."
The Liberal Democrats argue that they have been using the term "stakeholding" for some time, but remain to be convinced by Mr Blair because of the lack of detail.
One aide said: "It's a question of 'where's the beef?' - or rather, 'where's the steak?' "