Conventional theories of ethnic politics argue that political entrepreneurs form ethnic parties where there is ethnic diversity. Yet empirical research finds that diversity is a weak predictor for the success of ethnic parties. When does ethnicity become a major element of party competition? Scholars have explained the emergence of an ethnic dimension in party systems as the result of institutions, mass organizations, and elite initiatives. But these factors can evolve in response to an emerging ethnic coalition of voters. The author advances a new theory: ethnic cleavages emerge when voters seek to form a parliamentary opposition to government policies that create grievances along ethnic identities. The theory is tested on rare cases of government policies in Prussia between 1848 and 1874 that aggrieved Catholics but were not based on existing policies or initiated by entrepreneurs to encourage ethnic competition. Using process tracing, case comparisons, and statistical analysis of electoral returns, the author shows that Catholics voted together when aggrieved by policies, regardless of the actions of political entrepreneurs. In contrast, when policies were neutral to Catholics, the Catholic party dissolved.