The silver lining for Blair is the Tory collapse

A by-election win, however sensational, doesn't of itself send a party into the electoral Premier League

When the Liberals snatched their celebrated by-election victory in 1962, Timothy Birdsall, the cartoonist then used by the brilliant satirical programme That Was The Week That Was, drew on live television a map of the world which featured Orpington at its hub. Every intercontinental line on the Birdsall map - from Beijing, New York, Los Angeles, Sydney, and so on - led straight to the hitherto not very famous Kent suburb.

The point of the cartoon, of course, was to debunk the extravagant hype which had surrounded the Orpington "breakthrough" which commentators and MPs had rushed to insist heralded a whole new three-party era in British politics. In this Birdsall was prophetic. By the time it came to the general election two years later the Liberals improved their performance over 1959 by just three seats - bringing the total to nine.

Although the circumstances are far from identical, it's similarly premature to write Brent East into the history books as a momentous turning point. As the Liberal Democrats have more reason to know than most, a by-election win, however sensational, does not on its own propel a party into the electoral Premier League.

This isn't to belittle their victory. To come from third place and defeat Labour, for the first time in a by-election, is a signal achievement. To see its stand on Iraq -undoubtedlyone of the key issues at Brent, especially among the substantial ethnic minority population - rewarded is a source of real satisfaction. More widely, it demonstrates the extent to which Charles Kennedy's strategy has shed any lingering impression that the Lib Dems are a pale shadow of Labour.

And this is much more than a trick of the light. New research by Nottingham University's Philip Cowley and Mark Stuart shows that the Lib Dems' Commons voting behaviour has changed dramatically on a range of policies much wider than Iraq. By this stage of the first Blair parliament, the Lib Dems supported the Government in about half of all whipped votes. In the current parliamentary session, they have opposed the Government in 74 per cent of whipped votes. The Brent result is a handsome vindication of the Lib Dems' emerging role under Mr Kennedy as a bona fide opposition party.

But it does mean that the Lib Dems do need a streak of sobriety to offset all the euphoria. Mr Kennedy is wholly entitled to the triumphant reception that now awaits him at his Brighton conference next week. But to build on the success he now needs to use his keynote speech to start looking and sounding like a genuinely credible alternative opposition leader.

For the Government, the result is a serious psychological blow which will inevitably darken the clouds over Tony Blair as he approaches his own conference the week after. This was a test with real voters and the first Labour by-election defeat since 1988 cannot fail to have a real world impact. Blairites see themselves in a tunnel post-Iraq and pre-Hutton. Whether there is light at the end of it and New Labour can emerge - if not unscathed, since the Hutton findings on the handling of Dr David Kelly in the final period of his life can hardly fail to inflict damage - at least intact remains to be seen.

But until then the by-election will be used by union and party opponents of policies - including those on public-service reform - to press their case at Bournemouth on a resistant PM that a change of heart is needed. Particularly since the lamentable turnout underlines a deep disaffection with the political process as well as with their own government among many Labour supporters.

The silver lining for a shocked Labour Party, however, is the utterly dismal performance of the Tories. The argument which Iain Duncan Smith will use to the Scottish Tories today - that the Lib Dems have made a strategic error by attacking Labour from the left in ways which won't work in many of their (Tory-held) target seats - is rocky to say the least.

His criticism that the Lib Dems do not always say the same things in different places has something in it. But in their populist opposition to tuition fees the Tories are in the same camp as the Lib Dems. And on Iraq, at least one recent poll shows disaffection with the joint IDS-Tony Blair policy on Iraq as nearly as high among Tory supporters as among Labour ones.

In normal times you might say that Labour would be worried about the Tories becoming so crushed that they might finally decide to change their leadership to one closer to the mainstream of public opinion. And while that danger can't be ruled out, there is no sign that the Conservatives are capable of overcoming their disarray enough to agree an alternative leader.

What's more, the massive Labour to Lib Dem swing at Brent doesn't necessarily spell doom for the governing party. For even if the Labour vote is badly squeezed in the general election in all those seats where the Lib Dems are most active, the principal effect will be to takes seats off the Conservatives who hold most of the Lib Dem target seats.

If nothing else, the by-election is a dramatic curtain-raiser for the conference season in which all the parties will need to guard against overreaction but may well fail to do so: excessive euphoria at the Lib Dems'; tension, turbulence and volatility at Labour's; and depression and paralysis at the Tories'.

d.macintyre@independent.co.uk