Having patiently deconstructed since 1992 the myth that the Liberal Democrats, as presently constituted, would ever have sustained in power a Tory government, he has seen real results from his arms-length alliance with Labour. He secured a change in the electoral system for the European elections, seats on a joint Cabinet committee on constitutional reform and, above all, thanks to tactical voting, a once unthinkable 46 seats in last year's general election. But without also winning a more proportional system for electing the House of Commons, he will have failed to carry off the glittering prize that would allow him honourably to form a future coalition with Labour; in which case, it's hard to see why he would want to stay in his present job.
The forces, moreover, are already marshalling to ensure that his career ends in just such a noble defeat. Outside the ranks of political junkies, electoral reform comes close to zero in the league table of subjects the voters are interested in. But its capacity to cause trouble is almost unlimited, because it affects the issue of prime concern to most politicians: their own careers.
It is true that some Labour MPs genuinely believe in the fairness of a first-past-the-post system. But to mobilise effective opposition to reform, these idealists have to rely on the naked self-interest of those MPs - mainly in safe seats - who simply fear for their survival in a system that would reduce the number of constituency MPs in all parties and increase the number of Liberal Democrats at their own expense.
And to judge by yesterday's BBC survey of MPs there are quite a lot of them. Nor are they alone. The Cabinet is divided. In an interview with this newspaper in 1995, Blair indicated he might allow ministers to campaign according to their individual preferences in a referendum on PR. But is it really credible that he would recommend change without securing the agreement of Gordon Brown, not to mention other sceptics like Jack Straw or John Prescott? Hardly.
Finally, Blair has to overcome his own multiple doubts. He shares Lord Jenkins' frustration that the division between Labour and the Liberals at the beginning of the century delivered most of the rest of it to the Conservatives. Electoral reform would indeed give Labour the opportunity of doing the opposite in the next century. On the other hand, having changed Labour into a party that appeals in its own right to the middle ground, he has always instinctively mistrusted PR as the quick fix to power.
His friend Romano Prodi, the Italian prime minister, never misses an opportunity to press on him what a mess a proportional system has made of government in his own country. And then there is the matter of the referendum itself - which no grown-up politician, including Ashdown, thinks would be a pushover.
Neverthless the ice is beginning to crack. Lord Jenkins will, in all probability, recommend at the end of next month an electoral system that helps to institutionalise tactical voting by giving voters a second as well as a first preference in electing their constituency MPs and includes some form of top-up to make the final parliamentary outcome closer than it is to the distribution of the party's share of the national vote - but not so close that the large majority of MPs does not continue to represent individual constituencies nor that an overall majority for one party is impossible.
This latter point is important, meeting, as it does, one of Blair's main objections - that permanent coalition gives small parties a disproportionate share of power. There has been a civilised wrangle between Lord Jenkins and the Prime Minister about how large the top-up component would be, with Blair determined not to make it too large. For a man who allegedly hasn't made up his mind about change, the Prime Minister has been taking an awful lot of interest in the conclusions of the Jenkins report.
Which is partly - though only partly - why Ashdown remains confident that Blair will, in the end, support the changes recommended by his eminent mentor. My hunch is that he is right.
The new system is highly unlikely to be in place by the next election. Jack Straw appeared yesterday even to qualify his earlier declaration that the plan was to have a referendum in the current parliament. It could, for example, take place at the same time as the next general election. But happen it surely will. And the opposition, while formidable, is not invincible. In the Labour Party, for instance, PR would offer the only means of rescuing some MPs currently occupying enemy territory since the extraordinary landslide of May 1 last year by giving them the chance to compete for seats on a party list when the high tide of Blairite popularity recedes in a second general election. So too with the Cabinet. Gordon Brown's best chance of succeeding Blair as prime minister, for instance, is for Labour to stay in office for the foreseeable future rather than be voted out of it.
If Ashdown is wrong, however, his own future will not be the only casualty. The Liberal Democrats will probably come off the Cabinet committee and start doing what many of its activists like best - attacking the Labour Party at every level. That's where the centrifugal pull of the party is - illustrated by an article by the MP Norman Baker in the current Liberal Democrat News calling for a little less of the "constructive" and a little more of the "opposition". To which the answer in many Labour quarters is a resounding "so what?".
So quite a lot is the answer. The Liberal Democrats are maddening in lots of ways. Seventy-six years out of power have made parliamentary opposition a proud way of life for many of the delegates gathering in Brighton. Much of the party rank and file also suffers from an ideological schizophrenia: anti-state but in favour of tax and spending; self-professedly to the left of Labour while eager to preserve the theoretical right to form a coalition with the Tories. Moreover, while Ashdown has heroically - and rightly - striven to reposition the Liberal Democrats as the thinkers of the centre and centre-left, the menu of policies that will be served up this week is neither as innovatory nor coherent as he likes to claim.
But the best of Ashdown's party - especially, but not only, the section that defected from Labour in 1981 - is precisely what's missing from the Government now. A politics which mobilises the popular anti-Tory majority will be a more relaxed politics; one that might not depend so much on moguls like Rupert Murdoch, who continues to exact a price for helping Labour to stretch the party's own constituency to its limits. The Ashdown- Jenkinsite fragment of the centre-left, pro-Emu, mildly libertarian, not starry-eyed about corporate power, gently redistributive, may be just what a long-term Blair administration needs to make it complete.
To start reuniting the centre-left is an aspiration the so-called New Labour loyalists who oppose reform should think carefully about before trying to sabotage it. It's not only a matter of what Labour can do for the Liberal Democrats. Much as they will no doubt exasperate the Government this week, it's also what the Liberal Democrats can do for New Labour.