Six San Francisco residents sued the city in federal court today to challenge its instant runoff voting system.

The lawsuit claims that the way the city implements the system is unconstitutional because some voters are denied the ability to have their vote counted in later rounds of balloting.

The instant runoff system, also known as ranked-choice voting, was approved by a voter initiative in 2002 and put into effect beginning in 2004 for the offices of mayor, Board of Supervisors, district attorney, city attorney, sheriff, public defender, treasurer and assessor-recorder.

The system is intended to avoid the cost and the risk of low voter turnout in having a separate runoff election at a later date when no candidate in a race wins a majority.

Under the system, voters can rank three choices in each race. If no candidate in a race wins a majority, the candidate with the lowest number of votes is eliminated and his or her votes are transferred to the second choices of each citizen who voted for that candidate.

The process continues until one candidate achieves a majority.

Some races, such as supervisor contests, sometimes have a dozen or more candidates.

The lawsuit claims the system violates the constitutional right to vote because voters whose candidates are eliminated in early rounds have no voice in the final rounds of ballot counting in the instant runoff.

The suit seeks a preliminary injunction that would require the city either to return to having a separate runoff election or to allow voters to rank all candidates in a race.

A hearing on the motion for a preliminary injunction is tentatively scheduled for March 12 before U.S. District Judge Susan Illston in San Francisco.

Matt Dorsey, a spokesman for City Attorney Dennis Herrera, said lawyers for the city had not yet seen the lawsuit, but said, “It’s the city attorney’s job to vigorously defend the laws voters enact, and that includes ranked-choice voting.”

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