Does populism threaten American democracy, and if so, what is the nature of that threat? In dialogue with the comparative literature on populism, this article considers the opportunity structure available to populist parties and candidates in the American political system. I argue that compared to most other democracies, the US system offers much less opportunity for organized populist parties but more opportunity for populist candidacies. Today’s major parties may also be more vulnerable to populist insurgency than at other points in US history because of (1) changes in communications technology, (2) the unpopularity of mainstream parties and party leaders, and (3) representation gaps created by an increasingly racialized party system. Although no democratic system is immune to deterioration, the US constitutional system impedes authoritarian populism, just as it obstructs party power generally. But the vulnerability of the major parties to populist insurgency poses a threat to liberal democratic norms in the United States, just as it does elsewhere.