While groups have been central to thinking about partisan identity and choices, there has been surprisingly little attention paid to the role of perceptions of the group composition of the parties. We explore this critical linking information in the context of religious groups, some of the chief pivots around which the parties have been sorting. Using three national samples, we show that perceptions of the religious group composition of the parties are often biased-evangelicals overestimate the presence of evangelicals within the Republican Party and the irreligious within the Democratic Party. The key finding is that individuals are far more likely to identify with the party in which they believe their group is well represented-a finding which clarifies the role of party image shifts in constructing partisanship, the limits of the culture war motif, and the importance of social perception in shaping beliefs about party representation.