Never at a loss for words - or wit

THE mahogany door opens on an office so densely green it appears to have been papered in moss from County Kerry. Through the door steps himself, talking. Sen. Daniel Patrick Moynihan is a man never at a loss for words, as his new book proves.

It is titled simply ''Loyalties,'' and there are 96,000 carefully chosen Moynihan words in this slender book, down from the senator's standard manuscripts of nearly 192,000 when first sighted by his publisher.

The book deals with foreign policy and focuses on traditional loyalties - to international peace, international racism, and international law. It landed with a hefty thump on the desk of publisher William Jovanovich of Harcourt Brace Jovanovich Inc., who said it should be cut in half. ''He said it was going to be short, or it wasn't going to be read,'' remembers Senator Moynihan, with a fey smile that makes him look like the tallest leprechaun in the world.

Brushing a persistently wayward lock of silver hair off from his forehead, a habit that punctuates his conversation, he admits, that ''. . . It's true, I think, of a couple of feet of thick books which have had only 2,300 readers. What I do know is that people who have got this book have read it, really read it.''

And no wonder. It is a provocative, articulate, and highly ethical book, written with the stiletto wit that is a Moynihan trademark:

''There was a catatonic quality to the decision to deploy the MX, an absence of will; a leakage of reality now past the point of return,'' he writes in the first of the three essays which makeu up the book. Titled ''Launch on Warning,'' it traces the steps leading up to what he views as the US abandonment of loyalty to the tradition of deterrence as a principle of American nuclear policy.

Can the MX be stopped?

''Oh, sure,'' he says. ''I think I can say the event isn't final until those missiles go down those tubes. . . . It hasn't happened until it's happened. It's one Congress off, one president off, it's a two-, three-, four-year horizon.''

He has been talking and senatoring since 8 o'clock in the morning, and it is now nearing 8 in the evening. So he takes a break and crosses the room to get a cold drink for himself and this interviewer. In the ''talking heads'' format in which senators frequently appear on TV, Moynihan with his round, ruddy face and sometimes cherubic smile looks like a smaller man than he is. In person he is formidable: tall as an NCAA basketball player and lean as a knife. On his face he wears a bemused expression.

This is after all the senator who, chagrined over the soaring costs of the new Hart Senate Office Building, introduced a resolution a few years ago on the subject of the plastic sheathing that had covered the building during winter construction. The resolution suggested that since removing the sheathing exposed ''a building whose banality is exceeded only by its expense,'' the sheathing should be restored.

Wit and resonance have marked the public career of Daniel Patrick Moynihan, the former Harvard professor of government who has served four presidents and has been ambassador to both India and the United Nations. When Moynihan, a Democrat, served as President Nixon's adviser on domestic affairs Time columnist Hugh Sidey described him as standing out in that administration ''like a volcano in a cornfield.''

In the Senate, Moynihan is vice-chairman of the Select Committee on Intelligence. ''This room has to be swept regularly'' for bugs, he says cheerfully, glancing around the high-ceilinged office, with its cream marble fireplace. On the opposite wall hang two stunning paintings on loan from the Hirshhorn Museum, where he is a trustee.

''Loyalties'' is Moynihan's twelfth book. Speaking of his chapter on international racism, Moynihan points out that ''democratic assumptions we have comfortably lived with up until the last quarter century don't work as assumptions anymore and have to be defended. We had thought the case for democratic systems had been made,'' he continues, using Israel as a case in point and tracing a Russian campaign to equate Zionism with racism. Moynihan, the author of an embattled Senate resolution to move the US Embassy from Tel Aviv to Jerusalem, has a strong Jewish constituency in New York. He says, however, he has no loyalty to Israel, as such.

The chapter people have the most trouble with, he says, is the one on international law: ''The Idea of Law in the Conduct of Nations.'' He rumbles like a thunderhead on the premise of that chapter: ''That there was once something called international law, . . . nations were bound by it. . . . And we once really did take this very seriously.''

He chides both the Carter and Reagan administrations, writing that neither ''display(s) a sense of the past American commitment to the role - if not the rule - of law in world affairs.'' On the ''Sovietization'' of American foreign policy he concludes: ''For the United States to respond in kind is a policy devoid alike of ethical authority, political promise, or legality.''

The book was written during the Senate recess last August, when Moynihan and his wife, Elizabeth (also a writer), were vacationing on their farm near Pindars Corners, N.Y. She raised herbs, while he raised Cain at the typewriter in their 100-year-old former schoolhouse.

How does one man with time-consuming jobs like being senator or ambassador manage to turn out 12 books? ''I was taught by a master, that great writer Paul Horgan,'' he says. Moynihan dropped out of LBJ's Great Society in 1965 to attend the Wesleyan University Center for Advanced Studies, and from Horgan he learned the discipline of writing daily. When he is on a book, Moynihan lashes himself to the typewriter each day from 9 a.m. to 1 p.m.

There is one thin booklet he keeps with him, like a talisman, and it's not his own. In the middle of a statement on law he bolts up and over to his desk, saying, ''I find I walk around with a copy of the Constitution. . . . You think you know what's in it, but you don't. It's so dense.''

He thumbs through his worn, tan copy of the Constitution and reads aloud from it, like a Shakespearean actor relishing a passage from ''Hamlet.''

One of the most important things any president can do, he says, ''is to see that the Constitution is safe, that that which we have agreed to in all its penumbra is as secure as I can make it, and (he must pledge that) 'while I'm here what we have agreed to will be upheld.' ''