--- - |2 The established consensus in political behavior research is that discrimination by political institutions motivates marginalized groups to vote and protest their conditions. However, existing studies miss a comparison between states with high and low levels of political discrimination, and they miss a comparison between states before and after the development of opportunities for groups to mobilize. In particular, a growing body of research shows that sexual‐minority groups face discrimination to varying degrees across Europe. Sexual minorities in states with high levels of discrimination lack the support of other minority‐group members, which encourages political participation. The analysis is based on surveys of 30 European countries, conducted before and after the 2004 European Union enlargement, which provided a stronger political‐opportunity structure for sexual minorities in Eastern Europe. Eastern Europe and Western Europe provided contexts with relatively high and low levels of sexuality‐based discrimination, respectively. In Western Europe, those who report sexuality‐based discrimination exhibited higher levels of participation, in comparison to those who did not report discrimination. In Eastern Europe, those who report sexuality‐based discrimination exhibited lower levels of participation before the 2004 enlargement, but they did not exhibit these lower levels after the 2004 enlargement. - Political Psychology, Volume 39, Issue 5, Page 1013-1030, October 2018.