It's ye olde "blank check" argument -- easy to understand, and perhaps even persuasive.

CNN: Borger: Old arguments just don't work

Mr Alexander, who was a spokesman for the now defunct Britain in Europe lobby group before becoming an MP in 2005, rejects the "blank cheque" argument, saying the pre-manifesto document is merely part of the normal process the party goes through every time it produces a manifesto.

BBC: Manifesto Man: Danny Alexander