Political scientists have acknowledged the importance of nationalism as a constitutive element of radical-right politics but have typically empirically reduced the phenomenon to specific out-group sentiments. Sociologists, in contrast, have devoted more attention to theorizing and operationalizing nationalism but have only sporadically engaged in debates about institutional politics. The present study brings these literatures together by considering how nationalist beliefs shaped respondents’ voting preferences in the 2016 U.S. presidential election and how the election outcome built on long-term changes in the distribution of nationalism in the U.S. population. The results suggest that competing understandings of American nationhood were effectively mobilized by candidates from the two parties, in both the 2016 primaries and the general election. Furthermore, over the past 20 years, nationalism has become sorted by party, as Republican identifiers have come to define America in more exclusionary and critical terms and Democrats have increasingly endorsed inclusive and positive conceptions of nationhood. These trends point to the rising demand for radical candidates among Republicans and suggest a potentially bleak future for U.S. politics, as nationalism becomes yet another among multiple overlapping social and cultural cleavages that serve to reinforce partisan divisions and undermine the stability of liberal democratic institutions.