The pollster and commentator Peter Kellner has written an open letter to Nick Clegg suggesting that to save himself in 2012 he backs an In/Out referendum, he argues that the Yes side would win because Cameron would join Ed Miliband and Alex Salmond in campaigning for a Yes. Kellner believes, somewhat optimistically, a united political class would overcome entrenched popular opinion and press opposition.

They would, in Kellner’s view, sway currently €uro-sceptic public opinion in the course of the campiagn to the Yes side. Given that Clegg’s negative public approval rating has gone from a terrible -40% to a catastrophic -55% this would for once put Clegg on the public’s side of an argument. Kellner argues that if Clegg called for a referendum he would receive the public’s credit, Cameron would have to concede. Tory backbenchers would demand it and Cameron would not be able to resist the Tory right and the LibDems combined.

Kellner is a technocratic pollster and this has a certain Machiavellian logic to it. Guido has one other reason why Clegg should call for an In/Out referendum: he promised one without any Cameron cast-iron “if the treaty isn’t signed” wiggle room or qualification.

Clegg promised in writing

The Liberal Democrats believe that Britain should have a referendum, not on the narrow question of adopting this new treaty, but on the big question that faces Britain, whether we remain a part of the EU or not.

The question of Britain’s place in the European Union has poisoned our national politics for decades.  As a nation, we need to lance this boil and decide once and for all if we want to be a part of this European Union or not.

So he can outflank the Tories on Europe and restore some of his lost trust. Why is he waiting to honour his promise?