Cornell Law professor Michael C. Dorf comments on a recent concurrence by Justice Clarence Thomas in a case in which the Court vacated as moot a federal appeals court ruling that the president cannot block users’ access to his Twitter account. Professor Dorf explains why Justice Thomas’s reasoning is deeply flawed, but he points out that Justice Thomas’s conclusion that the First Amendment might permit Congress to forbid Twitter from moderating content on its site finds unlikely support in arguments historically put forth by progressive politicians and scholars. In their view, very large private actors who exercise power over people’s lives comparable to and sometimes even exceeding that of government should be subject to the same sorts of norms that the Constitution applies to the government.