Mayoral Elections in the UK use the Supplementary Vote (SV) electoral system. This system is employed to elect single office-holders for a whole regional or local authority area (spanning many constituencies or wards, and sometimes called ‘at large’ elections) while taking account of the preferences of as many voters as possible. It has been very successfully used to elect the powerful London Mayor since 2000, and all the other elected mayors in England. In 2012 it was employed for the first time to elect Police Commissioners in England and Wales.
The Supplementary Vote keeps the tradition of X voting in local areas. But now people have a ballot paper with two columns on it, one for their first choice and one for their second choice. They put an X vote against their chosen candidate in the first preference column, and then (if they wish) an X vote in the second preference column.
The key difference in the SV system from First Past The Post (FPTP) is in what candidates need to do to get elected. The first preference votes are counted and if anyone has more than 50% of the votes cast then they are elected straightaway, and the counting ends. However, if no one has majority support then the top two candidates go into a runoff stage, and the candidates placed third, fourth, fifth etc are all eliminated at the same time. The second preference votes of people who voted for one of the eliminated candidates are then looked at. If any of these voters cast a second choice vote for either of the two candidates still in the race then these votes are added to their piles. Whichever of the two top candidates now has the most votes then wins.
This process of knocking out low ranked candidates and redistributing their voters’ second choices ensures that the largest feasible number of votes count in deciding who is elected as the mayor or police commissioner. It does not always completely guarantee that the person elected has a majority of votes cast. But in repeated London elections the winning mayor has had nearly three fifths support amongst votes counted – a very clear result that greatly enhance the legitimacy of the office-holder.