From the late 1930s through the mid-1950s, the Congress of Industrial Organizations (CIO) brought together America’s working men and women under a united class banner. Of the 38 CIO unions, 18 were ‘left-wing’ or ‘Communist-dominated’. Yet the political struggle between the CIO’s ‘Communist dominated’ and right-leaning unions was immensely divisive and self-destructive. How did the Communists win, hold, and wield power in the CIO unions? Did they subordinate the needs of workers to those of the Soviet regime? The authors provide testable answers to these questions with historically specific quantitative analyses of data on the CIO’s origins, internal struggles, and political relations. They find that among the CIO unions, the Communists were more egalitarian, the most progressive on class, race, and gender issues, and leading fighters in struggles to enlarge the freedom and enhance the human dignity of America’s workers.