THERE'S NO SUCH thing as a "nice presidency," or at least there's no such thing for long. By "nice" we mean all those things to which the Boy Scouts pledge eternal fealty, plus a kind of political noncombatant status allowing the incumbant to assert, declare and promulgate policies that are -- well -- nice. Good-heartedness and exemplary rectitude and self-evidently benign intentions are, unfortunately, not proof against the pressures and maraudings and testings of others. On the contrary, often as not they invite the attentions of the other, less well brought up, beasts in the jungle. President Carter should be considering these things.

This has been a full rich week for him, a lot of pushing and shoving. In the details, anyway, it is no more certain what the Russians have been doing in Cuba than it is that Sen. Kennedy will run. But challenges tend not to come in absolutely clear, certain and full-fledged form. That's the hell of it, and that's what makes them particularly tricky challenges. The president, in short, is being assaulted and pushed, but in a measured and selective way, by his most feared political competitor at home and the country's most feared political competitor abroad. Whatever else these developments may mean, they surely reflect an assumption about the condition of the Carter presidency -- that it is malleable and weak

Often the Carter presidency is defined and defended by its admirers as a manifestly high-purposed enterprise that would score many more worthy achievements if only the distractors and disturbers and plain predators would lay off so it could go about its business. That business, under this construction, in turn gets defined as the fulfillment of an agenda of rationalization and reform -- making things as right and equitable as they can be, by deciding what should be done and asking the appropriate instruments of government to do it. The trouble is that what are viewed in this light as the spoiling diversions and side issues are, on the contrary, the essential business of the presidency itself.

In practice and in his pronouncements on the meaning of the office, Jimmy Carter has insisted otherwise. He would in a very precise sense of the old saying, rather be right than be president, insofar as being president involves power-playing and engagement in a continuous series of struggles by which the authority of his office is maintained, renewed and asserted. The fight to get to the White House, in other words, is actually only the beginning of it and guarantees nothing thereafter beyond room and board.

No one will know for a while how either the Russian-Cuban or Kennedy challenge will play out, how Mr. Carter will respond and how well. But the pressures have this in common: they test not Mr. Carter's "will" in the romantic sense in which that world always comes into play when the heat is on a prsident, but rather the very nature and conception of his presidency itself as he has defined it.We are about to see in more ways than one whether the rationalistic, low-key, what's-all-the-fuss-about and -- yes -- rather passive and mechanistic presidency of Jimmy Carter has the strengths that he and its other promoters say it has.