The faithful are justifiably elated, but where to now?

Liberal Democrats gather this week in good heart after their outstanding victory in Brent East. But once the immediate elation of victory has passed, the party has some important questions to face. What does it want to achieve? And how does it mean to achieve it?

The history of Liberal Democrat electoral strategy is a story of inconsistency. In the 1950s and 1980s, the party set out to replace Labour as the principal opposition to the Conservatives. In the 1990s, "constructive opposition" was born and for a while Liberal Democrats even sat on a cabinet committee with Labour ministers. And then over the past two years the party has talked of replacing the Conservatives as the principal opposition to Labour.

Liberal Democrat strategy has seemed to alter every time the electoral wind has changed direction. When Labour have been in the doldrums, it has dreamt of becoming Britain's main party of the left. When the Conservatives have been in trouble, it has hoped to become the main force of the right.

Both strategies are unrealistic. Even in 1983, when the SDP/Liberal Alliance was just two points behind Labour in votes, it won 186 fewer seats. And given the way the parties' votes are currently distributed, the Liberal Democrats might have to be as much as six points ahead of the Conservatives before they overtook them in seats. While the Liberal Democrats have substantially improved their ability to turn votes into seats, the first-past- the-post electoral system still presents a barrier to their ability to replace Labour or the Conservatives.

But there is another, more realistic, goal the party could achieve. That is to become the sandwich between Labour and the Conservatives. Consider, for example, what might happen if at the next election Labour's share of the vote fell nine points to 33 per cent, the Conservatives' share increased by just four to 37 per cent, while the Liberal Democrats gathered the remainder of the spoils with a five point increase to 24 per cent.

Repeated across the country, such a pattern would be enough to remove Labour's majority - but still leave the Conservatives far short. To govern, Labour would have to start talking to Charles Kennedy, not because they wanted to be nice to him, but because they would need his 59 votes.

And we may not be so far from such a world. YouGov's most recent poll puts the Conservatives on 37 per cent. Labour's current tally of 35 per cent is only two points above our assumption, and the Liberal Democrats are just four points below.

The Liberal Democrats need to maximise the damage they inflict on Labour, and then ensure that they - and not the Conservatives - gather most of the spoils. And what the Brent East by-election signals is that the party may now have the opportunity and the potential to do so.

In achieving this position, Mr Kennedy has been ably assisted by the recent difficulties of the Labour Government, and the continuing difficulty the Conservatives appear to have in persuading voters they are ready once more to take the reins of government. But he has also helped himself.

While his stance on the Iraq war was doubtless born of principle, it has also left the party better placed than the Conservatives to profit from the doubts that have emerged on the wisdom of the war. Meanwhile, Mr Kennedy has also dropped the last vestiges of "constructive opposition". This does not just free him to criticise Labour as much as he wants. It also raises the prospect that Liberal Democrat supporters who backed Labour to keep the Conservatives out will now no longer do so. Such a change could cost Labour dear.

But discontent about Labour's record on public services needs to be matched by ideas for doing better. The party has already decided to drop its distinctive policy of more tax and spend. Instead, it has opted for decentralisation. Whether that message can be sold effectively remains to be seen.

John Curtice is Professor of Politics, Strathclyde University