Donna Brazile has confirmed Bernie Sanders’s worst suspicions. Ms. Brazile, who served as interim chairman of the Democratic National Committee during the fall 2016 campaign, says in a new book that during the primaries, the DNC was controlled by Hillary Clinton’s campaign. Ms. Brazile claims the arrangement was “not illegal,” but that is far from clear.

Ms. Brazile reports that when she arrived on the job in July 2016, Gary Gensler, the campaign’s chief financial officer, told her the DNC was fully under the control of the...