"It is not a referendum on Nick Clegg nor David Cameron. It is a referendum on AV."
But the comparative evidence shows that doesn't always happen - with voters often expressing a view about the government of the day. Many Labour supporters who don't have strong views about AV one way or the other want to protest the Coalition government and its cuts agenda. The polling evidence suggests that they could decide the referendum.
But it is impossible to vote against the Coalition on AV.
Andrew Rawnsley set out in Sunday's Observer the case that a Yes vote would cause deeper trouble for David Cameron than a Yes vote would for Nick Clegg.
In Independent political editor Andrew Grice's analyses those referendum dilemmas, and e quotes me setting out why a vote to spite Nick Clegg would make George Osborne very happy indeed.
A No vote cast to spite Nick Clegg for forming the Coalition is a vote that bolsters George Osborne, the cuts and his strategy for a Tory majority in 2015 on the existing voting system," said Sunder Katwala, general secretary of the Labour-affiliated Fabian Society. "The one result that would really rock the Coalition would be big Liberal Democrat losses in the local elections combined with a Yes vote on AV. That would see Mr Cameron facing a furious revolt from his backbenchers and grassroots Tories, as the man who failed to win a majority last May and who had now lost the electoral system they love too."
I expand on the argument on how much chief Tory election strategist Osborne believes he can gain from Nick Clegg's unpopularity in a post for LabourList this morning.
The post echoes arguments made about Osborne's future plans also made by some of the best informed centre-right commentators, such as The Telegraph's Ben Brogan.
This helps to explain why the Chancellor of the Exchequer has suddenly found a great deal of time to complain about the Electoral Reform Society campaigning for electoral reform.
But Osborne's plans depend on Labour votes in the referendum. Do those thinking of voting to spite Nick Clegg want to give Osborne's Tory campaign a vital political boost?