President Trump's decision to fire over a dozen agency inspectors general may be legal, but he has made other moves that are almost certainly unlawful under existing Supreme Court precedent, Humphrey's Executor in particular.
On Monday, Trump purported to fire two Democratic members of the Equal Employment Opportunity Commission and the Chair and General Counsel of the National Labor Relations Board. Any one of these dismissals could result in litigation, and one in particular could set up a direct challenge to the Humphrey's Executor precedent.
Under Humphrey's Executor, decided in 1935, Congress may prevent the President from removing members of multi-member independent agencies (such as the Federal Trade Commission) without cause. A more recent decision, Seila Law (which I unpacked here) held that this does not apply to agencies exercising substantial authority headed by a single individual (such as the Consumer Financial Protection Bureau). While Seila Law did not purport to modify Humphrey's Executor, the two decisions are clearly in tension.