Wisconsin was relatively tolerant of third parties in those days: The left-wing Progressive Party elected several state officials in the '30s and '40s, including a governor and a senator, before the party folded itself into the GOP in 1946. (The Republicans were more ideologically diverse back then.) Thanks mainly to a large local contingent of German leftists, Milwaukee had been a Socialist stronghold since the early decades of the century. The party's first mayor, Emil Seidel, served from 1910 to 1912; the second, Daniel Webster Hoan, held the title from 1916 to 1940. He was knocked out of office by Zeidler's brother Carl, who was not a Socialist, and whose chief claim to fame was to have the pulp writer Robert Bloch running his campaign. Bloch's friend Harold Gauer recalls: "most of the active Socialists were old parties with white underwear showing over the tops of their high-button shoes, with chewed cigars sticking out of their choppers, who had no idea of how to appeal to young people -- who didn't know how to stop printing doctrinaire tracts and do something innovative. And when guys like myself and Bob Bloch, who depicted the city hall as a 'House of Mystery' and a lot of other whoopee stuff, came along, they ran into something that could only happen once in a lifetime, and they were lost."
Milwaukee's "sewer socialism" was never particularly radical. Indeed, in the first quarter of the century, Milwaukee Leader editor Victor Berger was the head of the party's right wing. Obviously, "right wing" is a relative term when you're discussing the Socialist Party; I'm following the spectrum laid out by James Weinstein in The Decline of Socialism in America, which stretched from Berger's reformists on the right to the IWW's revolutionary syndicalists on the far left.
Berger was also a virulent racist. According to Weinstein, he
proclaimed that only by keeping the United States a "white man's" country could socialism be victorious, and at one point went so far as to appeal for the defense of white womanhood against the invasion of "yellow men." Early in the debate Berger warned that the United States was already beset with one race problem and that if something were not done, "this country is absolutely sure to become a black-and-yellow country within five generations."
That may be part of the historical background to one of the more surprising coalitions of 1924, described in Kenneth Jackson's The Ku Klux Klan in the City:
The political situation in Milwaukee led to a curious alliance between the Socialist party and the Klan. Through the common bond of anti-Catholicism, the two groups supported John Kleist for the state supreme court as both a Socialist and a Klansman. The Klan had more Socialists on its rolls, Kleist told party leaders, than they did.
I'm not sufficiently familiar with the Milwaukee politics of that era to say how much that reflected bigotry among the Socialists, and how much it reflected "progressive" ideals within the Klan.
Unlike Berger, Zeidler was, to his credit, a vocal supporter of equal rights for blacks. Indeed, his politics were largely indistinguishable from those of the left wing of the Democratic Party; the sewer socialists built a lot of public housing and other municipal projects, but they never made a move towards, say, workers' control of the means of production. Their most welcome legacy was to make Milwaukee a place where politicians can be a bit more maverick than is the norm on the national stage. Even after it went back to electing Democrats, the city had room for pols like John Norquist, mayor from 1988 to 2003, a "fiscally conservative socialist" who quoted Jane Jacobs, supported school choice, spoke at the Cato Institute, and crusaded against freeways and modern architecture. He was eventually felled by a sex scandal, proving that even a Milwaukee mayor can behave like a conventional politician.