When liberals worry about losing a major Supreme Court case, they usually make appeals to the Court’s “legitimacy.” This is intended to attract Chief Justice John Roberts by suggesting that a conservative outcome would damage the institution’s reputation. The ritual is disingenuous but usually subtle.

Five Democratic Senators have had it with subtle. In a remarkable and threatening amicus brief, Sheldon Whitehouse, Mazie Hirono, Richard Blumenthal, Richard Durbin and Kirsten Gillibrand all but tell the Justices that they’ll...