As often seems the case today, American politics in the 1850s were nearly all-consuming and stubbornly tribal. So it was hard—and bitterly so—for hundreds of thousands of Northern Democrats to abandon the political organization that had long formed the backbone of their civic identity. Yet they came over the course of a decade to believe that the Jacksonian Democratic Party had degenerated into something thoroughly autocratic and corrupt. It had fallen so deeply in the thrall of the Slave Power that it posed an existential threat to American democracy.

Placing the sanctity of the nation above the narrow bonds of party, these Democrats joined in common cause with former Whig antagonists in the epic struggle to save the United States from its own darker instincts.

Today, a small but influential cadre of Republican elected officials, strategists and policy experts faces a similar choice. Heirs of Ronald Reagan, they have grown to believe that their party has also degenerated into something ugly and undemocratic—hostile to science and fact, rooted in an angry spirit of racial and ethnic nationalism, enamored of foreign strongmen and hostile to American institutions, and so fundamentally estranged from the nation’s founding values that it poses an existential threat to American democracy.