The leadership of the Marin County Republican Central Committee and Democrat Toni Shroyer, who plans to challenge District 5 Supervisor Judy Arnold, are attempting to place an initiative on next year’s November ballot that would impose term limits on county supervisors. The initiative, which needs 11,418 signatures to make it onto the ballot, would impose a three-term limit on all supervisors for a maximum of 12 years on the board, consecutive or not. Those behind the petition say term limits will encourage civic participation, curb the influence of campaign contributions and drive the supervisors to focus on “service to the county above that individual’s career.” The ordinance would apply retroactively,  “purging those who have already reached the new term limits,” the petition says. West Marin’s District 4 Supervisor Steve Kinsey, the board’s most senior member, would be one of those prohibited from running for reelection: he was elected to his fifth term in 2012, winning 63 percent of the vote. Ms. Arnold, the board’s president, said term limits are unnecessary. “I have always believed that term limits are up to the voters at the ballot box,” she said. “If they are not happy with their supervisor, they can vote them out.”