Some commentators have suggested that the Logan Act is a dead letter under a legal doctrine known as “desuetude,” which holds that a law can become unenforceable after a lengthy period of disuse. Only two people have ever been indicted under the statute, and neither of those indictments resulted in a conviction.

However, no federal court has ever refused to enforce a law because of desuetude. In 1964, a federal district court specifically held that the Logan Act remains valid. Quoting Shakespeare’s “Measure for Measure,” the court said of the statute: “The law hath not been dead, though it hath slept.” And the Supreme Court referred to the statute as recently as 1991.

Others have argued that the Logan Act cannot be enforced because it violates the First Amendment’s free speech clause. But the Supreme Court rejected a similar argument in a 2010 case called Holder v. Humanitarian Law Project.

The court held that the freedom of speech must give way to a federal statute banning “material support” for foreign terrorist groups. The court’s rationale — that citizens do not have an absolute First Amendment right to aid a foreign entity adverse to United States interests — would seem to apply regardless of whether the entity is a terrorist group or an often-hostile foreign state.

Why, then, have so few cases been brought under the law? Often prosecutors do not bother to bring cases against defendants whose wrongdoing poses little danger. Very few private citizens have any hope of persuading a foreign country to change its United Nations votes. Mr. Flynn, however, was no ordinary private citizen but the incoming national security adviser. He would soon be in a position to reward Russia for cooperating with the Trump transition team.

Another reason that few cases have been brought under the Logan Act may be that citizens who conspire to thwart United States foreign policy rarely do so in the open. Richard Nixon, as a candidate during the 1968 campaign, almost certainly violated the statute when he sought to sabotage the Johnson administration’s peace overtures to North Vietnam, but details of Mr. Nixon’s effort did not emerge until after his death.

“Nixon did it,” never much of an excuse for a president, thus doesn’t work here. Indeed, Mr. Nixon’s sabotage of the peace talks illustrates the importance of the statute and the potentially dire consequences of its violation.

If Mr. Flynn violated the Logan Act, then so did the “very senior” official who directed his actions. If that official is Mr. Kushner, then Mr. Kushner could go to jail. If it is Mr. Pence or Mr. Trump, then impeachment proceedings could be in order. To be sure, the Republican-controlled Congress, not Mr. Mueller, decides who to impeach. But if the phrase “high crimes or misdemeanors” means anything, it includes violation of a serious criminal statute that bars citizens from undermining the foreign policy actions of the sitting president.