How do electoral institutions affect self-identified partisanship? I hypothesize that party registration acts to anchor a person's party identification, tying a person to a political party even when their underlying preferences may align them with the other party. Estimating a random effects multinomial logit model, I find individuals registered with a party are more likely to self-identify with that party and away from the other party. Party registration also affects voting in presidential elections but not in House elections, leading to greater defection in the former where voters have more information about the candidates. These insights illuminate varying rates of electoral realignment, particularly among southern states, and the makeup of primary electorates in states with and without party registration.