But yesterday's announcement is ultimately an admission that his strategy for going beyond that is not working. He wanted a realignment of British politics - but it was a realignment that seemed superfluous to an electorate already realigned behind Tony Blair and which seemed undesirable to his party.
He tried to change his party, but in the end it proved too resistant to change. In the past few weeks, there have been signs that he had given up trying, as the depth of hostility among Liberal Democrats to working with the Labour Government has been increasingly evident. He was forced, for example, to vote against last November's Queen's Speech, which contained virtually nothing with which the Lib Dems disagreed. Mr Ashdown's decision to go is a public declaration of his loss of faith in his own party.
But in the end his failure was to change the Labour Party, which has made its hostility to working with him equally evident. For all his success in establishing his party as the second party of local government, and in sending more MPs to Westminster than at any time since the Liberals were overtaken by the Labour Party earlier this century, he could go no further without achieving a breakthrough in changing the electoral system.
After the Labour landslide removed his bargaining power, his strategy for obtaining proportional representation was fatally flawed. He never really decided whether he wanted to be part of a broad, progressive, liberal coalition - in which case electoral systems are only a means to an end, and changing them was rendered pointless by Mr Blair's redrawing of the contours of the political landscape - or whether PR was desirable as an end in itself.
Could he have played it differently after Mr Blair's unexpectedly decisive victory? Yes. He could have stuck to an independent path, defending liberal principles against the incipient authoritarianism of New Labour, and arguing for a plural political system for its own sake. Whether that would have been any more successful must be doubtful. The failure of Roy Jenkins's compromise PR system to catch the public imagination was nearly decisive; Mr Blair's shelving of the prospect of a referendum on electoral change before the next election closed the door to the early prospect of breaking the mould. In retrospect, however, Labour's win in May 1997 will be seen as the event that sealed Mr Ashdown's political fate.
The challenge for the Liberal Democrat party is to prove that that election has not sealed its fate, too. In its euphoria at winning 46 MPs, the party may have been temporarily blinded to the significance of what was happening: that Mr Blair was realigning without them. And since then, its opposition to Mr Ashdown's strategy has been based on tribalism. If the party had stood up for what it believed, supporting the Government's budgets and opposing the illiberalism of its policies on education, crime and terrorism, instead of chafing at any element of co-operation, it would be in a stronger position to choose a successor able to build on the impressive platform which Mr Ashdown bequeathes to his ungrateful party.