But to understand why, we need to start with some tribal history. Since the first neolithic Liberal activist emerged from a dank cave full of bison bones (or since the party stopped winning general elections, at any rate) the Liberals, and then Liberal Democrats, have been committed to a particular kind of voting system, called STV. This is proportional - a long word for fair - and complicated.
It requires large constituencies, from which up to five MPs are elected. And Liberals love it, overwhelmingly, unconditionally, beyond reason - rather like small girls love Barbie and Germans love beer.
Similarly, every good Liberal is taught, at an early age, to fear and detest another kind of voting system, known as the Alternative Vote, or AV. In brief, this means that no candidate can win a seat without having more than half the votes; every voter getting second and third choices to be redistributed if necessary.
To you, dear reader, this may not seem particularly shocking. But Good Liberals regard it as akin to incest, or reading the Daily Mail. Because AV is not really proportional, or fair, they see it as a swindle. So the news that leading Lib Dem parliamentarians are signalling their interest in AV is like finding Ian Paisley crouched in a confessional.
So what are they up to? They are having private conversations with the Labour Party which may lead to a dramatic reshaping of the British political system.
Up to now, the centre-right of British politics has kept itself together as ''the Conservative Party'', while the centre-left has often been divided. This simple fact has shaped 20th- century Britain - its hierarchies of power, its distribution of wealth, its distinctive and unusual class system. The eclipse of the Liberal Party by Labour, and later Labour splits, helped to ensure that, despite the achievements of the post-war Attlee government, this has been fairly called ''the Conservative century''.
The great question for Tony Blair and Paddy Ashdown is whether they can change the basic chemistry of party politics to fuse the centre-left and divide the centre-right. Proportional representation might have that effect. If Blair believes there is a pro-welfare state, pro-Europe majority in the country, he could, just possibly, unite it under his personal leadership in a way that the Tories would then find very difficult to shift.
Even without that, both opposition parties see the advantages of campaigning on the theme of ''all Britain against the Tories''. No one is seriously suggesting an electoral pact. But a campaign which saw Labour and Lib Dem politicians repeating similar policy ideas and arguments - and avoiding attacks in one another - would convey a new message.
One version of such a deal would work like this. Blair, already committed to a referendum on voting reform, accepts that he cannot go through a general election without saying whether or not he favours change. He takes a deep breath and says that, on balance, he thinks we do need a fairer system, thus demolishing the biggest remaining policy difference between the parties.
But which system might he change to? STV would mean redrawing every constituency boundary. It would be fiercely fought not only by the Conservatives but also by very many Labour MPs. The other main system is a ''list'' system, which continues with single constituencies but adds extra Members from central party lists and is detested at Westminster because it would create two classes of MPs.
Though there are endless hybrids and variants, the other main system is AV. Its effect would be very different from STV. One projection of the 1992 results by the London School of Economics implies very little change. Under the AV system, the Tories would have had 11 fewer MPs than John Major actually won; Labour would have had one less and the Lib Dems would have had 10 more.
Under STV, by contrast, the Tory tally would have fallen by 80, and the Lib Dems would have rocketed up. Such figures are speculative and not universally accepted - Vernon Bogdanor, who has devoted more time to the subject than almost anyone, disputes the very modest AV projections.
But however you play, it the difference between AV and STV is dramatic, the difference between redecoration and demolition. During the past two attempts to change the system, in 1917/1918 and in 1931, the pro-change case fell partly because of bickering between supporters of AV and STV. This time, change would probably be dependent on Labour and the Liberal Democrats agreeing which system they could live with.
Hence the significance of any Liberal Democrat tilt towards the once-despised AV. They may hope that once Blair has started moving, his own logic will drive him further. But they seem prepared to do business. One reason is given by a Liberal Democrat candidate writing in aparty magazine, The Reformer.
''There are very few seats where we come a good second to the Labour Party, suggesting that if we don't make significant gains this time, it is going to be an awful lot harder next time. ... There appear to be very few seats that will easily fall our way by attacking Blair,'' he wrote.
There would be no talk of a formal coalition or of cabinet seats for Liberal Democrats. But the party would urge voters to vote tactically against the Conservatives, and get similar support from Labour, thus maximising the anti-Tory mood. And if Blair won, the Lib Dems would support him in Parliament on issues including Scottish home rule, a bill of rights and perhaps education reform, while helping to offset the Labour left on issues like Europe.
There are plenty of senior Labour people who view such a deal with deep suspicion. After years of trying to keep the lid on Labour's spending plans, for instance, Gordon Brown would not relish trying to defend Liberal Democrat expenditure commitments, which he is known to regard as wildly over the top.
But Blair is clearly intrigued by the thought of spreading his ''project'' widely enough across the spectrum to give him 10 or 15 years in power rather than a Labour leader's usual four or five. He talks eloquently about pluralism. He has focused his objections to voting reform on the possibility of extremist parties gaining seats, and on support for the single-member constituency: both of these are objections which, as he must know perfectly well, could be answered by AV or a list system.
He is, in short, doing some serious thinking. Blair knows that the balance of probability is that he can win on his own; that he doesn't need to take this risk. But he also knows that the potential rewards are enormous. And as the man said, flashing an immodest smile at a press conference last week, he's always believed in upping the stakes.Reuse content