U.S. District Judge Richard Seeborg issued a summary judgment last week dismissing a challenge filed by six citizens, including former San Francisco Board of Supervisors candidate Ron Dudum.
Seeborg rejected the citizens' claim that the system, also known as ranked-choice voting, violated their constitutional voting rights.
"No matter how or for whom a voter casts his or her preferences, every ballot is counted and every ballot affects the election," the judge wrote.
In April, Seeborg refused to grant a preliminary injunction blocking use of the system in future elections. The summary judgment expands on the reasoning of the previous decision and is the final ruling in the case at the trial court level.
Jim Parrinello, a lawyer for the plaintiffs, said they are considering an appeal, but no decision has been made.
"We're disappointed, but the ruling wasn't a surprise because the judge had refused to grant a preliminary injunction," Parrinello said. "We respectfully disagree with the decision and think there are grounds for appeal."
The instant runoff was approved by city voters in 2002 and implemented in 2004. It is intended to avoid the cost and possible low voter turnout of separate runoff elections.
Under the system, voters can rank three choices in each race. If no candidate wins a majority, the candidate with the lowest number of votes is eliminated and his or her votes are transferred to the second choice of each resident who voted for that candidate. The process continues until one candidate achieves a majority.
The plaintiffs claimed the system was unfair because in races with more than three contenders, voters whose candidates are eliminated in early rounds have no voice in the final rounds of ballot counting.
Some city races, such as supervisor contests, have a dozen or more candidates.
Seeborg said that while the limitation to three choices "does exert some burden on voting rights, it is not severe."
"Even if a voter does not rank a single candidate who survives into the final rounds, he or she still participates in and affects the election," Seeborg wrote.
The judge said the city had proved that the three-choice limitation was "necessary to serve what are unquestionably important governmental interests: the orderly, accurate and user-friendly implementation of instant runoff voting."
The instant runoff applies to elections for the offices of mayor, supervisor, district attorney, city attorney, sheriff, public defender, treasurer and assessor-recorder.