Put it down to the political equivalent of having one’s knee tapped with a hammer: The Tories automatically lash out against the Alternative Vote, and promise to campaign against it, seeing it as something that will benefit the Lib Dems. I’m not sure they’re right.
The theory has always been that Lib Dem and Labour voters will transfer to each other. Yet the Con-Lib coalition has now turned that assumption on its head. There will be many Labour voters, fearing that a Lib Dem MP will return a Tory-Lib Dem coalition, who will just refuse to give a second preference to the Lib Dems.
Secondly, there are now Lib Dem voters who are looking at David Cameron in a different light, and will look at certain pro-Coalition Tory MPs that way too, and will give second preferences to them in a far higher pattern than would have expected in pre-coalition times.
Thirdly, don’t discount the Irish experience: Fianna Fail’s vote went up and the Progressive Democrats vote went down, leading to PD candidates acting effectively as “sweeper” candidates sweeping PD second preferences to Fianna Fail and electing them. It happened the other way too (coalition voters tend to give second preferences to the devil they know) but if the Lib Dems fail to keep their first preferences ahead of Tories in enough constituencies to be able to benefit from Tory second preferences, they’ll be hammered. As AV in Australia has shown, you need a hefty vote in the first place to be able to benefit from the system. You could actually have the Lib Dem vote rise nationally, but fall in their target seats, and see them win less seats than under first past the post.
Finally, don’t forget that both the Tories and Labour will finally be able to benefit from the refurn of “prodigal son” second preferences, from UKIP in the Tories case, and the BNP and Greens in Labour’s case. These votes will be vital in tight races, especially as AV will absolutely wipe out transfer-toxic parties like the BNP in terms of actually winning seats, even more than FPTP. Ironically, the BNP may end up helping Labour by bringing old Labour voters to the polls who would not have voted otherwise, and then vote Labour 2.
AV could change everything, but not the way the Lib Dems hope.