The unhinged reaction from the left to Kyle Rittenhouse’s acquittal last week is easily dismissed as a glimpse into the rancid minds and rabid hearts of the ideological brigands of modern progressivism. But it may offer clarity about the real threat posed by the Democratic Party and its fellow travelers to the values and institutions that have made America the most successful democratic republic in history. With varying degrees of apoplexy, from President Biden on down, the party’s leaders and its allies emphasized their dismay with or rage at the decision by 12 of Mr. Rittenhouse’s peers, after due deliberation,...