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 of sectarian zealots. After Margaret Thatcher’s Conservatives surged into power in 1979, Labour responded by moving left. Its 1983 election platform—promising centralized economic planning, unilateral nuclear disarmament, and the introduction of a price-control commission—was so extreme that critics dubbed it “the longest suicide note in history.” After the party lost in a historic landslide to the Conservatives, major factions within Labour still insisted that the party wasn’t left-wing enough. The Labour politician Edward “Red …