The choice of proportional representation (PR) is rarely included in work on American local politics. Yet we have long known that 24 cities adopted the single transferable vote form of PR from 1915 to 1948. Breaking with a machine–reform dichotomy that dominates the PR historiography, I investigate two partisan hypotheses about PR’s origins. One concerns the emergence of third parties. A second involves splits in ruling parties. In at least 15 cases, PR choice involved an alliance of convenience between ruling-party defectors and local minority parties. Evidence includes narratives on the partisanship of elite PR backers, comparison of case history and precinct-level referendum outcomes for three similar cities, and aggregate data on big-city charter change referenda from 1900 to 1950. New in this article is comparison of PR adopters with non-adopters. Party splits in places with sizable out-parties emerge as a distinctly American path to proportional electoral rules.¹