Big shift in Tory tactics with dawning horror that referendum could go horribly wrong

It wasn't meant to come to this. Six months ago, the Prime Minister only intended to play a walk-on part in the battle over AV.

He would make a keynote speech against the evils of abandoning first past the post and then stand back and let the Yes and No campaigns fight it out until May 5.

Not long ago the complaint among Tory MPs was that their leader was soft-pedalling to give Nick Clegg a bit of a lift. But now the heat is really on. Mr Cameron has made three speeches, given interviews, written articles and sanctioned a hard-hitting campaign against AV led by Foreign Secretary William Hague and Tory chairman Sayeeda Warsi.

Chancellor George Osborne has been allowed to enrage Lib-Dem grandees like Paddy Ashdown by accusing the Yes campaign of taking tainted money.

Such a dramatic change of tactics is down to several factors, including a dawning horror in Tory circles that the referendum commitment entered into last May could all go horribly wrong for Mr Cameron if - as looks certain - there is a paltry turnout.

Senior Tories have always believed the polling evidence that most voters turn "No" after being exposed to the arguments. But what if those freshly-persuaded voters stay on their sofas, while a smaller force of motivated Lib-Dem supporters dutifully turns out?

In London, where the turnout is likely to be under 20 per cent, there is what is becoming known as the "tenth of a tenth" problem. It goes thus: One in 10 Britons lives in the capital, so how can a yes vote carry legitimacy if only 10 per cent of them actually turn up to vote for it?

As one senior Tory MP mutters, the PM will face "a serious party management problem" if he ends up the loser. More unsettling for the whips is dark talk of guerrilla tactics to defy the outcome if there is a narrow Yes vote in a low turnout of, say, below 40 per cent (a situation that would imply more than four in five eligible voters failed to back this major constitutional proposal).

In such circumstances, Mr Cameron would have to squander serious political capital to keep his promises to Mr Clegg.

The other reason he has come out fighting is that neither the No nor Yes campaigns have achieved much traction from their dull list of celebrity supporters. On a question this big, it takes real political leadership to get noticed.