Researchers disagree over the definition, measurement, and expected political consequences of American patriotism, a situation that is fueled by the absence of a strong theoretical research foundation. We develop and evaluate a new measure of national attachment that is grounded in social identity theory (Tajfel and Turner 1979), drawing on data from three distinct sources: two studies of undergraduate students and the 1996 General Social Survey (GSS). Confirmatory factor analyses provide clear evidence that national identity is distinct from other measures of national attachment including symbolic, constructive, and uncritical patriotism, and nationalism. National identity has a number of other good measurement properties when compared to existing measures: it receives equal endorsement from conservatives and liberals (unlike most other measures which exhibit an ideological bias), develops with time spent in the United States among immigrants, and most importantly is the only measure of national attachment to predict political interest and voter turnout in both student and adult samples, consistent with the predictions of social identity theory. In that sense, the national identity measure outperforms all other measures of national attachment and provides unambiguous evidence that a strong American identity promotes civic involvement.