Even in this most partisan and dysfunctional of eras, we can all agree on one thing: Washington is broken. Politicians take increasingly inflexible and extreme positions, leading to gridlock, partisan warfare, and the sense that our seats of government are nothing but cesspools of hypocrisy, childishness, and waste. The shocking reality, though, is that modern polarization was a deliberate project carried out by Democratic and Republican activists.

In The Polarizers, Sam Rosenfeld details why bipartisanship was seen as a problem in the postwar period and how polarization was then...