Hitherto it has been assumed that introducing the Alternative Vote would benefit the Lib Dems, hurt the Conservatives and leave Labour untouched. But with a Conservative-Liberal Democrat coalition in place, that assumption would no longer be valid.
Under the Alternative Vote, candidates are placed in order of preference. If no one candidate wins over 50 per cent, votes cast for those at the bottom of the poll are progressively transferred in accordance with their voters' other preferences until someone does pass the 50 per cent mark.
A candidate who comes second on first-preference votes can win if they are successful at picking up second-preference votes.
In a poll in The Independent before the last election, both Labour and (to a lesser extent) Tory supporters were most likely to give the Liberal Democrats their second preference. But rather more Lib Dem supporters preferred Labour to the Tories.
The figures suggested that if the Alternative Vote had been in place, the Liberal Democrats would have won 79 seats, rather than 57. The Conservatives would have won only 281, not 307. Labour would have been marginally better off with 262 instead of 258.
But now that the Lib Dems are in coalition with the Tories, their supporters may be more willing to back Cameron's party with their second preferences. This could be reciprocated by Conservative supporters – as is very likely if the two parties encouraged their supporters to vote that way.
Of course Labour supporters would now probably be less keen on backing the Liberal Democrats with their second preference.
All in all, perhaps 20 per cent more Tory supporters might give their second preference vote to the LibDems than did in our poll, while 30 per cent fewer Labour supporters do so. At the same time twice as many Liberal Democrats might prefer the Conservatives to Labour.
If voters had behaved that way in May the Liberal Democrats would still have gained most, with 83 seats. But the Conservatives might have won as many as 316; Labour could have had just 223.
Might some Conservatives now be inclined to drop their opposition to the Alternative Vote after all?
John Curtice is Professor of Politics at Strathclyde UniversityReuse content