'That is not going to happen, certainly within my lifetime,' the former Labour Cabinet minister, who left Labour in 1981 to found the Social Democratic Party, said.
Mr Blair was 'the man for the second half of the 1990s without any question', Lord Rodgers said, and he hoped Mr Blair would win.
Mr Ashdown yesterday reacted tetchily as Lord Rodgers became the third member of the 'Gang of Four' SDP founders to praise Mr Blair and urge closer co-operation between the Liberal Democrats and Labour.
Rejecting suggestions that his recent speeches were a response to Mr Blair's election, Mr Ashdown said there was a saying that: 'If a dog barks in the Middle East, British intelligence is behind it . . . if anything happens anywhere on the political scene it must be because of Tony Blair. Politics and life goes on . . . not everything that happens . . . is because of a single but important event.'
Lord Rodgers's endorsement of Mr Blair was the warmest yet from the 'Gang of Four' and the one that most directly questioned Mr Ashdown's long-term strategy.
Lord Jenkins of Hillhead has urged Mr Blair to ensure 'friendly relations' with the Liberal Democrats, while Baroness Williams of Crosby has called for a 'common programme' on key issues.
Lord Rodgers said, 'all the indications are that Blair could be a good Prime Minister, could give the country the leadership it needs, and have the authority John Major desperately lacks'.
However many Liberal Democrat MPs won seats at the next election, he said, 'nobody is expecting us to be the Government'. The ideal outcome would be a coalition 'with the Labour Party inevitably the dominant partner in terms of numbers'.
There was no significant gap of ideas between himself and Mr Blair, he said, although he suspected that Mr Blair was 'far ahead of his party', and he would not be rejoining Labour.
Mr Ashdown protested that he did co-operate with other parties, and 'the whole style of politics I want to create is where political parties work together on issues of common interest'.
Robert Maclennan, the Liberal Democrat MP who defected from Labour with Bill Rodgers, maintained that there was 'no evidence that Liberal Democracts are rattled. I don't think Paddy Ashdown is in the slightest degree rattled'.
Mr Blair was ahead of his party, and would find that his party dragged him back, he said.
In a rebuke to the Government over its U-turn on the Bosnia arms ban, Paddy Ashdown, leader of the Liberal Democrats, said lifting the embargo would make the position of United Nations forces untenable. Ending the ban would dictate a withdrawal, but with personnel - including 3,350 British troops - and equipment dispersed across Bosnia, military commanders at every level in Sarajevo said withdrawal was not practical.