@20, Seattle may be making Seattle history, but while elected socialists are rare, they're not *that* rare. The US's first socialist mayor was elected in 1898 and the first communist mayor in was elected in 1932. Milwaukee had Socialists and self-described socialist Democrats as mayors for the majority of the 20th century, and Burlington, Vermont had a string of socialist mayors from 1981 to 2012. And Vermont has a small-s socialist as a US Senator, and Los Angeles, Minneapolis, Milwaukee, Cleveland, Oklahoma City, and Washington, DC have all had Green Party members in citywide offices.
Wikipedia has a list
of socialist mayors, definitely concentrated in the early part of the 20th century. I'm not sure where to find a list of socialists on city councils, but Greens
aren't that uncommon, and even managed to get a majority of council seats in a few crunchy granola towns.