Fears of high-density housing over the hill led to a three-term county supervisor’s crushing defeat by a challenger and another incumbent supervisor’s reelection by a scant 194 votes at the latest count in last week’s election.

Supervisor Susan Adams, who represents San Rafael, was unable to muster much support—only 39.9 percent—against San Rafael City Councilman Damon Connolly. Although Ms. Adams was first elected on a slow-growth platform in 2002, her opponents painted her as a…