The PM said during the first 2010 leaders' debate that the Lib Dem policy was too expensive.
Danny Alexander and Lynne Featherstone are both vulnerable to Labour challenges.
It will become harder for the PM to insist he can succeed when the europhile and the europhobe both declare he will fail.
Having successfully transcended his party's left-right divide, the Scottish Secretary could be the man to hold the Lib Dems together after the next election.
The first real-terms increase since 2008 will make it easier for the Tories and the Lib Dems to argue that the trend is moving in the right direction.
Labour tribalists and the media would immediately demand that Miliband follow the PM and promise to govern alone after May 2015.
Any minority government would soon collapse as there would be no impetus for the Lib Dems to support the Queen's Speech.
The party's new 404 page features Ed Balls and the message "Just like Labour's plan for the economy, this page doesn't exist."
Local voters resent outside interference and Lib Dem activists will be encouraged to rush to their leader's defence.