Many commentators have been quick to conclude that the election produced a bad result for the Lib Dems. Despite jumping five per cent in overall vote share, the party won just 11 additional seats and succeeded in ‘decapitating’ only one of the more forgettable shadow ministers. In contrast, the Tories, stuck stubbornly on 33 per cent, gained 31 new MPs. This was certainly not the breakthrough the Lib Dems had been hoping for.

It would be a mistake, however, for Labour to take much comfort from such underperformance, and not just because the Lib Dems’ advance let the Tories in through the backdoor in at least eighteen seats. Worryingly, the trends underlying the 2005 results indicate that the Lib Dems could now pose a direct threat to a significant number of Labour MPs. And unless we take that threat seriously, our chances of a fourth successive majority could be significantly diminished.

In the past, the Conservatives were seen as more vulnerable to a Lib Dem challenge because Tories held a higher proportion of the seats in which the Lib Dems took second place. But that assumption no longer holds water. Since 2001, the number of Labour seats in which the Lib Dems are second has more than doubled from 50 to 106. And of the 98 marginal seats that the Lib Dems could take on a 10 per cent swing, there is now virtually an even split – 49 Tory, 46 Labour. Furthermore, in seats marginal to the Lib Dems, the Tories grabbed a swing of one per cent back from them. In contrast, Labour suffered a hefty swing of nine per cent in the Lib Dems’ favour.

It is tempting to dismiss these advances as temporary, built on borrowed Labour votes that will flood back once the shadows of Iraq and tuition fees have receded. But such complacency ignores evidence of a more durable shift towards the Lib Dems.

Many of the 2005 Lib Dem victories did not come out of the blue – they were built on solid progress in previous elections. They revealed an ability not only to secure swings on a scale seen normally only in by-elections, but to repeat the trick at successive elections. Indeed, in eight of the 12 seats Labour lost to the Lib Dems, they came third as recently as 1997. Cambridge and Hornsey and Wood Green are stark examples of where the Lib Dems mounted sustained local insurgencies and vaulted over both Labour and the Tories to win. Furthermore, the Lib Dems have clawed their way up from third in eight of the 10 most vulnerable Labour seats. In some constituencies, they have seemingly come from nowhere, displacing the Tories in seats that were previously textbook Labour-Conservative marginals.

Moreover, there are dozens of examples of once safe seats that risk becoming tomorrow’s marginals – 33 seats where the 2005 swing to the Lib Dems exceeded 10 per cent, and 18 with a swing of 15 per cent over the past two elections.

Issues like Iraq or student fees cannot alone explain such extraordinary results. Worryingly, there is evidence that some of these changes are due to Tory voters siding tactically with the Lib Dems to defeat incumbent Labour candidates.

If we find ourselves fighting on two fronts, there is a danger our majority will evaporate altogether.

The changed political landscape – where the Lib Dems and not just the Tories are now our challengers in many seats – is reflected in changed policy terrain, too. Before the election, we saw the Lib Dems sliding to the right on issues such as scrapping the New Deal and the Child Trust Fund. Now they have gone further still, flirting openly with new right policies like a flat tax. This process is likely to continue as the Lib Dems ruthlessly target tactical Tory voters, while maintaining a veneer of progressive rhetoric in an attempt to bank their support on the centre-left.

Looking towards the next election, our campaign strategy must evolve to take account of both the enlarged Lib Dem threat and their slide to the right. We must challenge the Lib Dems concertedly on their alleged progressive credentials and demonstrate that only Labour truly understands what is needed to achieve a progressive society and is capable of delivering radical, progressive policies.

It is only Labour that understands the hardship that would be caused by freezing the minimum wage – which the Lib Dems argued for. Or the pain that would be caused by consigning young people back to the scrap heap through the abolition of the New Deal, as they were pledged to do. We must expose their double standards of telling different messages to different audiences and ensure the progressive vote knows that their posturing on Iraq camouflages a hidden rightwing agenda.

But a renewed emphasis on progressive policies and philosophy, vitally important though that undoubtedly is, is not on its own enough to win back progressive voters who have drifted towards the Lib Dems. We need, too, to recognise the importance of good local organisation.

In many of the Lib Dem advances, local factors, in particular strong local organisation, have been at least as important as national policy. For example, the Lib Dems have succeeded in building on strong performances in local elections, and using them as a platform for challenging incumbents in parliamentary elections. Often, there is an ugly character to such campaigning, with candidates diverging from party policy and posturing opportunistically on local issues. But it is effective all the same.

To tackle this problem, we need to step up our efforts to reinvigorate local parties and to encourage more community campaigning. This means supporting local parties both in attracting new members and in encouraging existing members.