President Donald Trump is not reluctant to accuse people of treason.

On Sunday, Trump targeted Rep. Adam Schiff, D-Calif., chair of the House Intelligence Committee, proclaiming on Twitter that he wanted the California Democrat "questioned at the highest level for Fraud & Treason."

On Monday, he elaborated, musing that a "fake and terrible statement" by Schiff might just be grounds for his "Arrest for Treason?"

Trump's tweets are often over-the-top. But these were particularly heinous because they are inconsistent with a key provision of the U.S. Constitution, and call up the very concerns that motivated its drafting.

Treason is the only crime specifically defined in the U.S. Constitution:

"Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort."

That's from Article III, Section 3, which continues:

"No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court."

The narrow definition of treason is a central feature of American exceptionalism and a sibling to the First Amendment's protection of freedom of speech.

The founding generation was deeply committed to the idea of republican self-government, which could not exist if political leaders could define treason as they wished. They wanted to protect dissent and opposition.

As James Madison explained, "New-fangled and artificial treasons have been the great engines by which violent factions, the natural offspring of free government, have usually wreaked their alternate malignity on each other." For that reason, Madison continued, "the convention have, with great judgment, opposed a barrier to this peculiar danger, by inserting a constitutional definition of the crime."

But it remains true that, as the legal scholar Charles Warren wrote in 1918, "The elements of the law of treason are extremely simple, and yet are little understood by laymen, and even by practicing lawyers of the present day."

It's evident that there are two forms of treason, separated in the Constitution by that important little word, "or."

Levying war against the U.S. covers internal insurrections against the government. Before the Civil War, almost all federal cases fell under that category.

Levying war is well-specified idea. Ordinarily, it consists of actually assembling armed forces. As Chief Justice John Marshall put it in 1807:

"However flagitious may be the crime of conspiring to subvert by force the government of our country, such conspiracy is not treason. To conspire to levy war, and actually to levy war, are distinct offenses. The first must be brought into open action by the assemblage of men for a purpose treasonable in itself, or the fact of levying war cannot have been committed. So far has this principle been carried, that ... it has been determined that the actual enlistment of men to serve against the government does not amount to levying war."

It should be clear that political acts of the most unsavory or despicable sort - lying, leaking, spying - cannot by themselves be counted as "levying war."

The second idea, of "adhering to their Enemies, giving them Aid and Comfort," is somewhat broader. As the Supreme Court made clear in 1877, the key term is "enemies," a term that is limited to "the subjects of a foreign power in a state of open hostility with us." Open hostility refers to declared or open war.

During World War II, Nazi Germany would of course count as an enemy, and after the terrorist attacks of Sept. 11, 2001, and the enactment by Congress seven days later of an Authorization for the Use of Military Force, al-Qaida would be so counted as well (so long as a private organization can be seen as "a foreign power").

"Aid and comfort" are also words of limitation, consistent with the founding generation's insistence on allowing a lot of scope for freedom. Communicating intelligence would certainly count. So would selling munitions to an enemy, or delivering prisoners and deserters to them. Transferring goods to an enemy might also count as treason, even if those goods are not munitions.

Under federal law, treason is punishable by death. The founding generation sought to ensure that this most heinous of offenses would be narrowly limited, and would never be a basis for high-level attacks on, or threats to, political opponents.

It's important to say that Trump isn't alone in reckless and irresponsible use of the term "treason." That is something that some of his critics, including the former director of the Central Intelligence Agency, John Brennan, and former Massachusetts Gov. Bill Weld, have also done.

No one should doubt that it's perfectly legitimate for Trump to object in the most vociferous terms to the statements and actions of Schiff, or to those of any member of the House of Representatives. But for a president or any high-level politician to use accusations of treason as a political weapon is not merely an error of law. It is a betrayal of the U.S. Constitution.

This article was written by Cass R. Sunstein, a columnist for Bloomberg.