With the announcement of Senator Betsy Johnson’s run for Oregon governor as an unaffiliated candidate, there’s a risk that she will be a spoiler and help elect a Republican under the current plurality election rule. An ordinal electoral system such as the supplementary vote would prevent such an occurence.
The supplementary vote allows for only a first and second choice, making it easy for voters to understand and and election administrators to tabulate. If a candidate receives a majority of the first choice votes, that candidate is elected. If no candidate gets a majority of the first choice votes, the second choice votes for all candidates except the top-two vote winners are transfered to the top-two candidates. The candidate with the most votes (plurality) is then elected. It wouldn’t prevent an unaffiliated candidate or minor party candidate from being elected, but it would prevent such a candidate from being a spoiler.
Article V, Section 5 of the Oregon Constitution appears to allow the supplementary vote for Governor. It specifies: “The person having the highest number of votes for Governor, shall be elected.” So, the supplementary could possibly be implemented with ordinary legislation.
The simplest ballot design for Oregon might be to list all the gubernatorial candidates twice: once for first choice and again for second choice. If a voter leaves the second choice blank or marks the same candidate for both first and second choices, the effect would be the same as the current plurality electoral system.
The supplementary vote is currently used to directly elect some mayors in the United Kingdom, including the mayor of London, along with police and crime commissioners in England and Wales.
Wikipedia: Supplementary vote
Electoral Reform Society: Supplementary Vote