David Cameron and the PR trap

Responding to the threat of a peasants' revolt in the wake of the expenses scandal, David Cameron is sounding off like a radical reformer. But his proposal, given to the Guardian, that the sitting Prime Minister should go to the country every four years instead of becoming a 'cling on' like Gordon Brown, is small beer compared to the changes his opponents wish to see. They range from a wholly elected House of Lords to the end of the Royal Prerogative. Camo is nibbling at the edges, offering the prospect of a 'limit' to the Royal Prerogative.

The most serious threat to Cameron is that the public anger at dodgy expenses - and the threat of more independent candidates standing against sitting MPs - could lead to an unstoppable tide in favour of some form of proportional representation.

Alan Johnson, the Health Secretary, has outflanked Gordon Brown by calling over the Bank Holiday for the least radical form of PR - the replacement of first-past-the-post ballots with an alternative, transferable vote and a top-up system (known as AV-plus) to allow parties to have extra MPs according to their proportion of the poll.

Johnson denied he was putting down a marker for the leadership with his Times article proposing a referendum on AV-plus. But for a man who earlier ruled himself out, he knows his proposal will enhance his chances of replacing Brown, should the PM reach for the pearl-handled revolver after June 4.

Calling for a referendum on the same day as the next general election was a master stroke. It means that should Brown step down, it will be Johnson's unique selling point. If he becomes Prime Minister in the next six months, he could force through the referendum in time for the next election. Liberal Democrat backing and a fair wind from the electorate would ensure him overwhelming support and Cameron would be left whistling in the wind. All his bold calls for reform today will not stop the Tory leader looking like an anti-reformer, if there is momentum in favour of electoral reform.

For Cameron, PR is a trap. His senior advisers are warning him to remember that it was promoted by Peter Mandelson as part of the strategic plan before Blair came to power to ensure the centre-left had a majority in Parliament for the rest of the 21st Century. It will lead to coalition politics, sure enough, but it will also leave Camo facing electoral suicide if he doesn't stop a PR referendum in its tracks.

And he may have to deal with it sooner rather than later. If Brown sees which way the wind is blowing, he's likely to nick Johnson’s referendum proposal for himself.