For generations, experts have argued that American politics needs cohesive parties to function effectively. Now, many fear that strong partisan views, particularly hostility to the opposing party, are damaging democracy. Is partisanship as dangerous as it may seem? In answer, this book offers a nuanced evaluation of when and how partisan animosity matters in today's highly charged, dynamic political environment, drawing on panel data from some of the most tumultuous years in recent American history, 2019 through 2021. The authors show that although partisanship strongly influences political behaviors, its effects are conditional, not constant. It is most powerful when politicians send clear signals and when an issue is unlikely to bring direct personal consequences. In the absence of these conditions, other factors often dominate decision-making. The authors argue that while partisan hostility has degraded US politics-for example, by politicizing previously nonpolitical issues and undermining compromise-it is not in itself an existential threat. As their research shows, the future of American democracy depends on how politicians, more than ordinary voters, behave.