Using the formation of a lesbian and gay electoral constituency as a case, this article demonstrates how activists and party elites contest and construct collective identities and groups. Activist–party interactions produce identity-building feedback that recognizes some groups and identities and rejects others, creating conditions for people to see themselves as partisans. I call this process “constitutive group mobilization.” I find that, when party actors affirmed civil rights and libertarian constructions of lesbian and gay people and politics, mobilization was relatively bipartisan. Republicans’ emerging alliance with the Christian Right, however, brought activists to form the National Association of Lesbian and Gay Democratic Clubs, crystallizing civil rights as the dominant linkage to partisanship. These developments reveal how groups and identities form endogenously to parties rather than entering the party system as preformed entities with fixed interests and partisanship. Thus, the lesbian and gay case provides insights about group and identity formation previously overlooked in party and LGBT politics scholarship.