Preferential voting has been introduced in a number of proportional election systems over the last 20 years, mainly as a means to increase the accountability of individual politicians. But most of these reforms have been criticized as blatant failures. In this paper, we discover a genuinely new fact, which calls into question this negative evaluation. We show that preferential voting in a general election can operate as a stand-in internal primary election for top party positions. To do this, we rely on a unique data set from four waves of Swedish local elections, which includes every nominated politician in each of 290 municipal assemblies. We use a natural-experiment (regression-discontinuity) approach to estimate the causal effect of winning the most preferential votes on becoming the local party leader, and find that narrow 'list winners' are over 50 percent more likely to become party leaders than their runner-ups. Comparing across politicians, the effect of list winning is the strongest for competent politicians, who are also more likely to draw preferential votes than mediocre politicians. Comparing across municipalities, the response to narrow list winning is the strongest within unthreatened governing majorities, where voters also use the preferential vote the most frequently.