The Liberal Democrats and Single Transferable Vote

So the bargaining begins. Right now, the Liberal Democrats are considering an offer of cooperation from the Conservatives. So far the Conservatives have not come out with an unambiguous offer of cabinet seats but seem to be hoping for Lib Dem support of a Conservative minority government. Labour has also reached out to the Lib Dems offering themselves as a partner should talks with the Conservatives break down.

So what are the Liberal Democrats going to set as their price? The conventional wisdom is that they will push hard for at least a referendum on changing the electoral system from the current Single Member District system (SMD, aka "first past the post") to a more proportional Single Transferable Vote system (STV). There is a famous (and slightly humorous) explanation of the Lib Dem proposal by John Cleese here. In that video, he refers to STV as "proportional representation. While STV is more proportional than SMD it's not a straight transmission of the share of the vote into a share of seats (which the proportional representation systems used in most continental European countries more or less do). STV is used by Ireland.

What would happen to the UK if the Lib Dems got what they want and the referendum approved a change to STV? Well, it would likely benefit the Lib Dems! In STV, voters can rank order candidates in multi-member districts. If a district has 4 seats, the 4 candidates with the most preferences win seats (see links above for detailed descriptions of how that gets determined). In Ireland the three biggest parties tend to get slight bigger shares of the seats than their first place vote shares. The smaller parties get smaller seat shares and vote shares but the difference is typically far less than we observe in SMD. You can see a summary of the latest Irish elections (including a report of the seat share percentage and vote share percentage) here.

In practical terms such an electoral system would likely make the Liberal Democrats frequent participant in government coalitions. The two biggest parties, Conservatives and Labour, would be big losers in such change. Under the current system (since 1979) the winning party in the UK has typically won about 40% of the vote but well over 50% of the seats. This year the Conservatives only won about 36% of the vote and fell short of a majority of seats. Any system that imposes more proportionality on the system would be bad news for any party that thinks they have a regular shot at being the top vote getter.

Just playing around with some naive numbers here: In 1979, the year Thatcher swept to power, the Conservatives won 43.9% of the vote and won 53.4% of the seats. In a more proportional system many voters would likely transfer their votes to smaller parties instead but if we simply use the 1979 vote shares are guide we can see that Labour and the Liberal Democrats had more than 50% of the votes together. Imagine a British political history with no Thatcher. Alternatively, Thatcher would have had to get the cooperation of the more moderate Lib Dems. If STV (or PR) had been used in 1979, it's likely that either there never woud have been a Prime Minister Thatcher or she would have been hamstrung by the need to form a coalitoin with a more moderate party. That's the kind of impact we're talking about.