It’s far too soon to know what role Mr. Obama will play in history, let alone whether he can be compared to F.D.R., or, as he is most commonly, to John F. Kennedy. But it is perhaps time to look more closely at this label that attaches to him, and how it has been applied in the past.

The “cult of personality” is used in the pejorative. But recast as a different name — call it charisma — and, as Roosevelt and other examples show, it can be a critical element of politics and its practical cousin, governance. It just can’t be the only element.

“Today, attacks on the cult of personality seem really to mean attacks on the ability to make speeches that inspire,” Mr. Caro said in an interview. “But you only have to look at crucial moments in the history of our time to see how crucial it was to have a leader who could inspire, who could rally a nation to a standard, who could infuse a country with confidence, to remind people of the justice of a cause.”

Still, Mr. Caro adds a caveat: “That doesn’t always translate into a great presidency.”

So what does it look like?

Charisma, as defined by the early sociologist Max Weber, was one of three “ideal types” of authority — the others were legal, as in a bureaucracy, and traditional, as in a tribe — and rested upon a kind of magical power and hero worship. That definition was, of course, unsuitable for modern times, as one of Weber’s many interpreters, Arthur M. Schlesinger Jr., wrote in “The Politics of Hope.” Its use became metaphorical, as Mr. Schlesinger wrote, “a chic synonym for heroic, or for demagogic, or even just for ‘popular.’ ”