Political elections exemplify complex decision processes in human populations. Data of proportional elections show a striking feature at different spatial scales, across years, and for several democracies: when ranking the parties according to their number of voters, the amount of votes grows exponentially with the party’s rank. We develop a mechanistic mathematical model of birth and death of parties and voter grouping based only on word of mouth and not on political contents, close to neutral models used in evolutionary biology (Ewens sampling formula), or Hubbell’s model of species biodiversity. Data and model agree strikingly well. The model explains, for instance, the steady loss of big-tent parties in France and Germany by the increasing number of parties standing for elections. A cannibalism effect (parties/candidates at a given rank systematically withdraw votes from others) can be identified. The interpretation and consequences of the rational or lack thereof of voters’ choices for modern democracies are discussed.