As night follows the day, the rumblings of discontent with the performance of Ronald Reagan as president are finding a familiar response: Americans are too capricious, the media too critical; we give up too quickly on our presidents; this gravely weakens us in our dealings with the rest of the world, not to mention what it does to the president's power and influence for the rest of his first term.

If we toss Reagan aside two years from now, he will be the fifth American president in a row to have been hailed at first and then hassled, harried and finally hounded out of office.

What are foreigners to think?

Nothing good, probably. But that's not the point. When the alleged leader of the free world abruptly changes course, the opportunities for miscalculation multiply. The problem is real, and never mind that sharp swings in public opinion are in the nature of democracies.

So it's worth asking the deeper question: consequences aside, are we too quick to come down cruelly and unfairly on our presidents in a way that cripples their ability to lead at home as well as abroad? Or could it be that we have had a bad run of luck, or poor taste, in our choice of presidents? Among the many distinguished political scientists and commentators who hold the public (and the media) to blame is Henry Graff, a history professor at Columbia University specializing in the presidency. Writing recently in The New York Times, he expressed grave doubts about the future of a republic that throws its leaders so lightly over the side.

"The attacks on Mr. Reagan," Graff argues, "are the latest in the denigration of the presidency that has gone on steadily for two decades." In this, Graff sees an America that has lost faith in leadership itself--the faith traditionally shown by reelection.

He was disquieted by "the eerie picture" of three rejected presidents (Carter, Ford and Nixon) standing together at Anwar Sadat's funeral. This, he believes, "delivered a message to the world--of American fickleness and improvidence with political talent."

Talent? Richard Nixon surely had a talent for conception in making foreign policy, reinforced by the great conceptualizer, Henry Kissinger. But he had less talent and still less tolerance for working within the political system, or even the law. Though he won a second term overwhelmingly, he made himself an argument for not re-electing incumbent presidents.

Carter's self-inflicted wounds were countless. But his inability to inspire confidence owed much to a stubborn refusal to accommodate to the exigencies of survival in the Washington jungle about which he knew next to nothing on arrival. Ford was a stand-in, not even nominated for a job he never sought, until he was first appointed to it as Spiro Agnew's replacement.

Lyndon Johnson inherited the job as a result of tragedy; only in office was he granted his party's nomination. Never having ventured much into the world at large, he was never really comfortable in it. Dean Acheson, a wise man, used to say that it is "our sad destiny to put people in the presidency with no experience in foreign affairs."

Witness Ronald Reagan. Like Jimmy Carter, his previous acquaintance with government at the federal level was second-hand, remote and casual. But to con tend, as Graff does, that by threatening to add him to the list of four previous presidential rejects, the American public will be saying that "we no longer value continuity of policies" is to miss the point about Reagan.

It is, interestingly, Ronald Reagan's own continuity over the space of 18 months in office that is being called into question--and not nearly so much by those who didn't want him in the White House as by those who did. There is scarcely a sound so heart-rending nowadays as the keening of conservatives crushed in spirit by the way Ronald Reagan has departed from dogma on arms control, East-West relations and a general readiness to stand tall against the international communist conspiracy.

It escapes me what this has to do with "denigration of the presidency" or "public fickleness." It is the public-- or that part of it most attuned to Ronald Reagan in the first place--that has remained constant. What's being registered, in Reagan's sagging standing in opinion polls, is very largely disappointment with a performance that hasn't matched campaign promises.

And that says something about all three--the public, the performance and the promises. What it says is that if the political scientists want to get to the bottom of all this, they ought to worry less about the way we pick on our presidents and more about the way we pick 'em.