The recent rise in identification with American political parties has focused interest on the long-term dynamics of party support. Liberal commentators cite immigration and youth as forces that will produce a natural advantage for the Democrats in the future, while conservative writers highlight the importance of high fertility amongst Republicans in securing growth. These opinions are not based on demographic analysis. We addressed this omission by undertaking the first ever cohort component projection (up to 2043) of populations by American party allegiance, based on survey and census data. On current trends, we predict that American partisanship will change much less than the nation's ethnic composition because the parties are similar in age structure. Nevertheless, our projections suggest that the Democrats will gain 2-3 per cent more support than the Republicans by 2043, mainly through immigration, although the higher fertility of Republicans may eventually offset that advantage.