Like it or not, Paddy and Tony still need each other

When Paddy Ashdown was a young officer in the Special Boat Squadron one of his many challenging jobs was to drive - literally - torpedoes underwater towards their targets. It's not a bad metaphor for what he's trying to do now: something difficult, dangerous and below the surface.

Slowly, cautiously, the Liberal Democrat leader is trying to condition his hesitant, wary, party to the possibilities, and the realities, of the new political order. Large sections of Liberal Democrat mankind cannot bear very much reality, so much of his big conference speech tomorrow will no doubt be delicately coded. But the warning he will issue, that there are no risk-free options for the party, is part of a long, tortuous, and increasingly urgent educational process. It's time to get real.

When the party was on its knees after the Alliance imploded in the wake of the 1987 election, it was unimaginable that Lib Dems could win 46 seats, have secured proportional representation for the European Parliament, be looking forward to coalition partnership in the governance of Scotland and be sitting on a cabinet committee under the chairmanship of a Labour prime minister. Today, the great goal of electoral reform for Westminster, and a quantum leap in the party's size and power, is at least in sight, if not yet within grasp. Ashdown's message will surely be that the party now needs fresh clarity about what is and isn't possible.

Tony Blair, it must be emphasised, regards the question of which electoral system Britain uses as essentially of secondary importance. That's hard for some steeped in a Liberal tradition which elevates proportional representation almost to an end in itself. But Blair seems to have been emphatically clear about what he wants at the Downing Street dinner he had with Paddy Ashdown, Roy Jenkins and Peter Mandelson on June 12. He seeks nothing less than the reunification of the centre left, whose split into Liberals and Labour made the century a largely Conservative one.

This is revisionism on a breath-taking scale: it says, in effect, that the Labour Party was an aberration and ought not to have happened. But Tony Blair may yet decide that his grand plan does not require PR.

There are still ministerial hawks who insist that Blair can afford to resist electoral change for Westminster on the grounds that top Liberal Democrats will, in any case, have nowhere to go but to Labour. I think they are wrong. The momentum propelling the Prime Minister towards electoral change is too great. But the stance taken by these hawks is a reminder of how tough the negotiations on the cabinet committee will be. It also demonstrates how far purists among the Liberal Democrats will have to compromise if the talks are to succeed.

The first big step is the electoral commission which will shortly assemble to decide what new electoral system should be put to a referendum. It is increasingly possible that Blair will offer Lord Jenkins the job of chairing that commission; the affectionate respect he has for Jenkins is, if anything, deepening over time. And this would be a significant gesture to the Lib Dems; it would suggest that Blair was at least persuadable in favour of Commons PR. Jenkins himself would certainly ensure that the commission proposed a genuinely proportional system to put to the referendum. But what kind?

Peter Mandelson advocates the alternative vote, under which electors state their first, second and third preferences; this gives electors much more of a real choice; it's good for Labour, it's good for the Liberal Democrats - it would have doubled their seats in 1997. But there's just one problem. It isn't proportional in the sense that the outcome in seats bears no more relation to the national percentage votes than first past the post.

The possible, cloudy, basis for compromise is this: the Commission could recommend an alternative vote system, topped up by additional MPs drawn from party lists to increase the proportionality. The alternative vote could be enacted for the next election, and the rest left till the next parliament and a wholesale redrawing of constituency boundaries.

Enough of the voting system nerdery. The significance of this possible change is that it would require movements from Blair as well as the Liberal Democrats. It is conventional wisdom to assume that the Prime Minister holds all the cards and the Lib Dems are utterly dependent on his pluralistic munificence to make any progress. But ministers should be careful about patronising the Lib Dems.

Blair was right to invite them into the committee. As ministers are already finding, senior Liberal Democrats, who know constitutional issues inside out, are proving rather helpful in filling the gaps in Labour's knowledge. It was also in Blair's long-term interests to invite them in. It's easy to forget, given Blair's current enormous popularity and his huge majority, that he won with only 44 per cent of the vote. Only co-operation with the Liberal Democrats offers him the chance of heading a government which commands the support of a true majority of the electorate.

And what goes for the Labour Party, goes for the country too. On Sunday night Shirley Williams made a little speech which reminded you what an aching, irreplaceable loss to the Labour Party was her departure in 1981. There was just a passing reference to the "ruthless" treatment of Labour dissidents during the referendum campaign. But it made the point: imagine for a moment a political landscape without the Liberal Democrats, a world without any body but the Tories to keep Labour's authoritarian tendencies in check. Ask not only what Labour can do for the Liberal Democrats. Ask also what they can do for the Labour Party.