UK Polling Report has a fascinating account of a recent YouGov poll that attempts to answer the question of how the UK parties would fare under the Alternative Vote (which is being voted on in a referendum in less than a month), given current polling trends.
YouGov found that there would be no net change for the Conservatives, a slight decline for Labour, and a substantial boost for the Liberal Democrats. There would still be a Labour majority, but of only 34 seats instead of the 60 that the same poll projects under FPTP.
The poll involved giving respondents a picture of a ballot paper and allowed them to fill it out as much as they wanted with preferences. This is important because “Labour second preferences disproportionately go to the Green party, but given that the Green party will normally have already been eliminated in a count before Labour is, it’s actually their third or fourth preferences that count.”
Perhaps not at all surprisingly, fewer and fewer Labour voters will give the LibDems a preference at all now.
What this all boils down to is that in Con v Lab marginals the lower preferences of Lib Dems would help the Conservatives win seats from Labour, in Lab v LD seats Conservative lower preferences will help the Lib Dems win seats from Labour, in Con vs LD seats Labour lower preferences will help the Lib Dems… but Con losses there will be cancelled out by Con gains against Labour.