This book argues that the United States now has, for the first time in American history, a genuine two-party system, with two fully-sorted, truly national parties, divided over the character of the nation. And it is a disaster. It is a party system fundamentally at odds with our anti-majoritarian, compromise-oriented governing institutions. It threatens the very foundations of fairness and shared values on which democracy in the United States depends. The book tells the story of how American politics became so toxic and why the country is now trapped in a doom loop of escalating two-party warfare from which there is only one escape: increase the number of parties through electoral reform. As it shows, American politics was once stable because the two parties held within them multiple factions, which made it possible to assemble flexible majorities and kept the climate of political combat from overheating. But as conservative Southern Democrats and liberal Northeastern Republicans disappeared, partisan conflict flattened and pulled apart. Once the parties became fully nationalized—a long-germinating process that culminated in 2010—toxic partisanship took over completely. With the two parties divided over competing visions of national identity, Democrats and Republicans no longer see each other as opponents, but as enemies. And the more the conflict escalates, the shakier America’s democracy feels.