If David Cameron was to come out and call Brown a liar, it would lead every TV bulletin and be splashed across every front page. But there would be a serious risk of blow-back. Labour would try and claim that Cameron was behaving like a public school bully. The claim could do reputational damage to Cameron.
But it needs to be put into the public domain. No BBC interviewer, with the possible exception of Evan Davis, is going to feel comfortable accusing the PM or a Cabinet Minister of lying when they start spouting these claims: if the presenter has it wrong in any respect, it would be the end of their careers. So, the Tories need to put it into the bloodstream.
The best way forward is to have a member of the shadow Cabinet, calmly accuse Brown of lying. A clear statement that the Prime Minister should stop lying to the British people would, as the editorials in today’s papers demonstrate, resonate.