American midterm elections are strange beasts. In a political system whose structures and symbols are rooted in executive and judicial power, purely legislative elections begin to feel like undercard events. A nationwide contest to decide which Congress will crank its engine while appointees, regulators, and judges actually make the laws? It hardly seems worth the time or expense.

And yet often in US history, a national midterm — not a single event but thousands of simultaneous and interlinked struggles — can mark a more distinct watershed than the clamor and color of a presidential election.

The Civil War, we know, was triggered by the victory of Abraham Lincoln and the Republicans in 1860. But the revolutionary crisis we call “the American Civil War era” really began with one midterm (1854), when a new antislavery party swept the North, and ended with another (1874), when the last live embers of Reconstruction were stamped out by Southern violence and Northern retrenchment.