Mr Ashdown defended his decision to create a new "Lib-Lab" agreement with the strongest hint yet that the Prime Minister had pledged to hold a poll in this Parliament.
He rounded on critics of his closer co-operation with Labour with a declaration that the deal represented an "historic moment" to transform Britain's political landscape.
He admitted that Mr Blair still had to persuade those cabinet ministers opposed to reform, but predicted that within a year the Government would be ready to fulfil its manifesto pledge to stage the referendum before the election.
Mr Ashdown said Mr Blair was "intellectually and emotionally" committed to reform. "The Prime Minister has moved his position from being unpersuaded and hostile to PR, to warm and presumed to be in favour of it.
"That is not insignificant. There is still an excellent chance of getting a referendum this side of the next election. I believe the Prime Minister would like to see it too."
In what was seen at Westminster as an indication that Mr Ashdown agreed to the new deal only in return for strong assurances on electoral reform, he said that the co-operation made it "more likely" that change would be delivered.
Mr Ashdown today faces astormy meeting of his party's ruling Federal Executive when critics will try to overturn the deal. However, he said that the agreement did not stop Liberal Democrats at local level opposing corrupt Labour councils.
The attempt to extend co-operation could instead usher in a new age of a "progressive liberal centre" that could rule Britain for the foreseeable future.
"I believe we can assemble a progressive liberal centre in this country, we can put together a movement of parties that can work together," he told Radio 4's The World This Weekend.
"There's a slight logical inconsistency between those in my party who argue for proportional representation, but are not prepared to touch with a barge pole the kind of policies it would bring into operation.
"It does not, I think, reflect very well on the party if you ask people at the last election to vote for a more co-operative style of politics and then were not prepared to do it when they got their vote in the following Parliament," he said.
Simon Hughes, the party's health spokesman, renewed his opposition to the plans yesterday. He said "many people" in the party were unhappy with the deal and could trigger a special party conference to discuss it.Reuse content