Supplementary vote

The supplementary vote (SV) is an electoral system used to elect a single winner, in which the voter ranks two of the candidates in order of preference. In an election, if no candidate receives an absolute majority of first-preference votes, then all but the two leading candidates are eliminated and there is a second count. In the second count, the votes of those who supported eliminated candidates are distributed among the two remaining candidates, so that one candidate achieves an absolute majority.

It is a specific implementation of the contingent vote and instant-runoff voting. Under SV, voters express first- and second-ranked choices of candidate only, and, if no candidate receives an absolute majority of first-choice votes, all but the two leading candidates are eliminated. If a voter's first-choice candidate is eliminated but their second choice is one of the two remaining candidates, their vote is transferred to the second-choice candidate. The candidate with the most votes is the winner. This means that the winning candidate has the support of a majority of voters who expressed a preference among the top two, although not necessarily a majority of votes cast in the first count. 

The supplementary vote was used in all elections for directly elected mayors in England, including the Mayor of London, and in elections for police and crime commissioners, until 2022 when it was replaced by first-past-the-post voting (FPTP).

History and use
In the early 1990s, the Plant Commission was established by the Labour Party to recommend a new voting system for the Parliament of the United Kingdom. When the Commission reported in 1993, instead of suggesting an already existing system, it recommended the Supplementary Vote system, which they said had never been used anywhere. (Apparently the commissioners did not know that a similar system, Contingent voting, had been in use in Australia as early as 1892.) 

Although some commentators credit the invention of SV to Plant, according to others, it was the brainchild of the then Labour Member of Parliament (MP) Dale Campbell-Savours, who advocated and outlined it in an article for the New Statesman magazine that was published September 29, 1989.

In 2000, several districts in England introduced directly elected mayors. It was decided to use the Supplementary Vote for the election of these new mayors, including the Mayor of London, and for the election of police and crime commissioners across much of England and Wales. Supplementary vote was used for these offices from 2000 to 2022.

In the 2021 London election, a record 5% of first preference ballots were rejected, mostly because voters tried to select more than one first preference on a ballot with 20 mayoral candidates divided into two sections. (The 2021 winner did not take a majority of votes counted in the first round (2.5M) but did take a majority of votes that were marked with a preference for either of the two candidates running in the second count.) The then Home Secretary, Priti Patel of the Conservative Party, responded by ending the use of the supplementary vote in England in 2022, citing voter confusion with a complex system. However, critics, including the Labour and Green parties, argued that the wasted votes were due to ballot layout and that the change was aimed at benefitting Conservative Party candidates. They also claimed that the supplementary vote was effective in increasing multi-party participation and was popular among voters.

The histories of two-round voting and other forms of instant run-off voting may be seen as part of the history of SV due to their similarities.

Impact on factions and candidates 
The Supplementary Vote is said to encourage candidates to seek support beyond their core base of supporters in order to secure the second preferences of the supporters of other candidates, and so to create a more conciliatory campaigning style among candidates with similar policy platforms. SV is also likely to improve the chances of 'third party' candidates by encouraging voters, who wish to do so, to vote sincerely for such candidates for whom, under systems such as 'first past the post', they would be discouraged from doing so for tactical reasons.

These positive effects are moderated by the incentives SV creates for voting, in some circumstances, for only candidates from among the leading three.

Political scientists Colin Rallings and Michael Thrasher noted two flaws of SV:
 First, since the automatic dual-ballot nature of SV dispenses with any need for a runoff two weeks later – as often happens for, say, the election of the president of France – voters cast their second preferences without being certain of which candidates will make the runoff. Consequently, some second preferences will be declared invalid because they bear only preferences marked for eliminated candidates.
 Second, it is possible for the victor to fail to achieve an absolute majority overall, for it is not an obligation for a voter to cast a second preference, and even when a second preference is marked, the vote will be ineffective if it is cast for a candidate who does not make it into the top two, when the first preference is marked also for a candidate who does not make it into the second round.

Like any system that elects a single representative, including other forms of the contingent vote, SV is not a form of proportional representation. For that reason, if it were used to elect individual members of a council or legislature, it could be expected to produce the kind of one-party-dominant results that are produced by other single-seat systems like 'first past the post' (plurality). However, election of representatives by only a minority of a district's voters is less likely under SV than under FPTP.

See also
Ranked voting systems
History and use of instant-runoff voting
Tactical manipulation of runoff voting
Elected mayors in the United Kingdom
List of democracy and elections-related topics
Alternative vote
Alternative Vote Plus
Single transferable vote

Notes

References

External links
Democratic and Electoral Shifts in Queensland (PDF)
London Elects: How the Mayor of London is Elected
Electoral Systems Index: Sri Lanka

Contingent vote
Single-winner electoral systems
Preferential electoral systems
Non-monotonic electoral systems