The 25th Amendment is the ultimate constitutional "check" — a corrective mechanism for an American president who is physically or psychologically unable to lead. Most important, it grants legal authority to those closest to power — first, the vice president and Cabinet members, then members of Congress — to stage an intervention. At the very least, these individuals are authorized to call a temporary timeout if the president is judged unfit to govern.

Is America today in need of such an unprecedented intervention?

The amendment, ratified in 1967 after President John F. Kennedy’s assassination, was constructed to assure a smooth transition when a president becomes incapable of leadership. (Its vague wording leaves room for both physical and psychological justifications.) By the 1960s, the dangers of an incapacitated president were far greater than at the founding of our country. But arguably, the stakes have only gotten higher. With tensions flaring around the globe, there can be no doubt as to the fitness of the man or woman in possession of U.S. nuclear codes.

Pundits and politicians alike have called for the amendment’s implementation over the past few months. But it is both practically and philosophically a tool of last resort. Unlike impeachment, which is controlled solely by Congress, the 25th Amendment requires action by the majority of the president's Cabinet and potentially Congress. This means that even in today’s polarized climate, partisan removal is unlikely. In addition, the bar for diagnosing mental health conditions is quite high.

It’s crucial to note that having a mental illness does not automatically disqualify a person from serving successfully as president. Indeed, as a Duke University Medical Center study estimates, up to half of the first 37 U.S. presidents displayed clinical features consistent with mental illness at some point in their lives. Two of our 10 most respected presidents, Abraham Lincoln and Thomas Jefferson, displayed symptoms suggestive of depression and anxiety disorders.

So how would one diagnose a sitting president — if it is indeed even possible?

This is uncharted territory

For one approach, a bipartisan congressional caucus could empanel a team of experts to evaluate the president’s condition. But critics argue that mental health conditions are too ambiguous to reliably evaluate. Trying to address a president’s psychological disability, they say, could open the door to partisan use of the 25th Amendment. As Peter D. Kramer and Sally L. Satel argued in The New York Times recently: “The medical profession and democracy would be ill served if a political determination at this level were ever disguised as clinical judgment.”