Britain's electoral system, of course, makes it much harder for third parties to turn their votes into seats, and thus exercise influence in proportion to their support. At the 1992 general election just under 6 million votes won the Liberal Democrats exactly 20 seats. Labour, by contrast, won 271 seats - 13 times as many - with less than twice as many votes. This arithmetic makes each seat gained far more valuable to Mr Ashdown than it is to his two mighty rivals.
Many of those who wish to see a change of government at the next election may be unbothered by this argument. As long as the Conservatives are defeated, they urge, who cares who does the damage? But there are very good reasons for believing that a strong third-party presence in British politics is beneficial, not just in representing its own constituency, but in invigorating political debate.
The trouble is that with his decidedly declasse appeal, Mr Blair more easily occupies the traditional territory of the third party than any of his predecessors. There must be many Lib Dem voters who feel that they could now vote Labour without putting the nation in thrall to the unions or other big battalions. And in many ways they are right.
There are, however, some factors that they should consider. In the first place the Liberal Democrats continue to be the authentic voice of decentralisation in Britain. Their commitment to devolution and to localism has never wavered and they have kept the flame of reform alive where others have tried to extinguish it. Partly as a result of their nonconformist traditions the Lib Dems have also been the quirky party - the party that can debate decriminalising cannabis, where others shy away. The Lib Dems have an impressive record of championing issues which one or both of the other two parties have only later subscribed to.
In non-election mode, Mr Blair has acknowledged the validity and the separateness of the Liberal Democrats. Earlier this year he spoke about the need for proper dialogue between the parties of the centre and centre- left. He did not believe that Labour in power should say "that's it, we're the government", but rather should work "to achieve the broadest possible base of consent". In other words he would accept the need for an accommodation with another party.
This is a far-sighted attitude. But it is one that can easily be forgotten or neglected in the heat of battle or by the temptations of power. Labour would do well to remember that it can neither subsume the liberal tradition nor obliterate it. A Lib Dem victory in today's by-election would do no harm to New Labour and would be good for the pluralism of British politics.