He was the future once

Wow. We never quite thought that the TV debates could have such an explosive effect on this General Election campaign. But the dust has settled, and the Lib Dems seem to be establishing some kind of poll increase of around 7%. They are unlikely to finish the campaign above 30%. But a final share of the vote of 27% or 28% seems entirely possible.

This is devastating for the Tories. Their entire strategy relied on snatching the 30 or so Lib Dem / Conservative marginals in the South West and South East. That now seems in serious danger. The Lib Dems are simply not going to lose all the target seats for the Conservatives in Cameron's crucial list of 118 marginals he needs to get his majority of one. Many of them were already going to be difficult. The likes of Chris Huhne, for example, occupies a marginal seat in Eastleigh near Southampton. But candidates such as Huhne - and others in the Tories' crucial list of 118 seats - are often seen as "sticky". I didn't think they'd shift before Clegg's barnstorming debate performance. They certainly won't now.

So, the key change is that it now appears more likely that we will have a Hung Parliament. The strong Lib Dem polling is encouraging Cameron to claim that a vote for Clegg is a vote for Brown. A foolish strategy. The whole reason Cameron is behind is his failure to properly clarify and explain what a Tory vote means. What do you get if you vote Cameron? For too long, he has pitched what a Tory vote doesn't mean. It means you don't get five years of Gordon Brown. Nick Clegg is right to say that, in the post-Blair age, tha vacuousness simply doesn't wash anymore.

Cameron faces a big headache for the next debate. Attack Clegg and he risks attacking the most popular politician since Churchill. He also risk attacking him on the issues that he's tried to drag his party to the centre ground on. Too much bashing of Clegg's wishy-washy liberalism could turn off the very Lib Dem voters he needs to win in his marginal seats. He has to pull of the difficult trick of claiming on the need for decisive government. A real mandate. If he'd properly clarified the Cameroonian project, that would be easily done. But he's still colouring in the gaps. Still struggling to explain the "big society". He has been out-Cameroned by Clegg. He no longer feels new. The line "he was the future once" could well turn out to be the epitaph on Cameron's campaign for a decisive mandate.

In a sense, Cameron was a victim of his own failure to "fix the Tory roof when the sun was shining". For too long, Cameron's strategy relied on Brown and the Labour party self-destructing. Major's 1992 election victory couldn't be pulled off by Labour could it? No, it couldn't. But a Lib Dem surge will give Cameron a mighty headache between now and polling day.

Best strategy? Give Clegg enough rope to hang himself. 18 days is a long time in politics. Cameron needs a Clegg gaffe to recover the ground. He's unlikely to lay a glove on him otherwise. Just ask Tony Blair and Gordon Brown - they'd been dealing with it until January this year.

PREDICTION: Conservatives short of overall majority by 10