Do people form relationships based upon political similarity? Past work has shown that social relationships are more politically similar than expected by chance, but the reason for this concordance is unclear. Is it because people prefer politically similar others, or is it attributable to confounding factors such as convergence, social structures, and sorting on nonpolitical characteristics? Addressing this question is challenging because we typically do not observe partners prior to relationship formation. Consequently, we leverage the domain of online dating. We first conducted a nationwide experiment in which we randomized political characteristics in dating profiles. Second, we analyzed behavioral data from a national online dating community. We find that people evaluate potential dating partners more favorably and are more likely to reach out to them when they have similar political characteristics. The magnitude of the effect is comparable to that of educational homophily and half as large as racial homophily.