Even after a string of recent victories in congressional primaries by members of the Democratic Socialists of America, national Democrats appear slow to recognize that ideologues on the left are attempting a hostile takeover of the party. The DSA’s ascent comes on the heels of a yearslong effort by socialists and progressive activists to steer one of America’s two major political parties toward identity politics, isolationism, and hostility to capitalism.Democrats looking for a way to reclaim their party can find inspiration in the career of Neil Kinnock, who in the mid-1980s rescued the British Labour Party from a determined band...