LONDON -- Could anything short of electoral defeat ever persuade Margaret Thatcher, soon to enter her ninth year as prime minister, to give up politics? Among those who know her, there is usually only one answer. "No . . . unless something were to happen to Denis."

Her husband Denis, Thatcher often says, is her sounding board and her strength. He is her one true crony, the one she lets see her cry and grit her teeth. Their marriage, she says, is "a great love story, . . . the golden thread" that has made everything else in her life possible.

Although Denis Thatcher recently celebrated his 72nd birthday, a national crisis does not seem likely. Fit and trim, he often attributes his good shape, according to his daughter, Carol, to two things: "Gin and cigarettes."

Britain's First Gentleman does not dabble in drug abuse programs. No fashion plate for British designers, he generally sticks to dark suits and old school ties. He expressed no interest in touring the Kremlin with Raisa Gorbachev when his wife paid an official visit to Moscow last spring. In fact, he decided to give the entire trip a pass, and reportedly prefers not to truck with "pinkos" at any level.

So unobtrusive is Thatcher when his wife is on show that visitors sometimes even forget who he is. George Bush's presidential campaign book, "Looking Forward," contains a photograph of the U.S. vice president and the British prime minister standing outside the front door of her official residence. With them, the caption says, are Bush's wife, Barbara, and Thatcher's husband, "Norman."

Yet despite his own reticence, Thatcher's husband is a celebrity in his own right. He has been the subject of a hit play, "Anyone for Denis?" that was also made into a television special. It depicted him as a very common type of middle-class Englishman -- a boozy philistine, gulping "snorts" of gin behind his wife's back, and spending most of his time on the golf course.

The spoof correspondence from Denis to a golfing chum, "Letters to Bill," is a regular feature of the popular political satire magazine Private Eye. The missives of an archetypal British middle-class tippler, with a bossy wife and blunt, right-wing views, they are a hilarious commentary on governmental issues of the day.

In the "Dear Bill" code, Margaret Thatcher is always known as "the Boss," or simply "M," while her close political ally, Ronald Reagan, is usually "Hopalong." Homosexuals, especially in the Anglican Church, are "shirtlifters," and nonwhites are inevitably "wogs." Political acolytes are "smarmy," and Labor Party leftists are "smelly-socks." Denis, in the letters, spends much of his time bending his elbow, and complaining about the loss of the good old days, when people knew their place and the empire was intact.

Far from causing outrage, Denis Thatcher's many caricatures have brought him no small measure of public affection. One theory is that his alleged lapses make him a welcome antidote to his wife, the nation's Iron Lady. Even the Conservative Party apparently thinks the laughable image of Denis is an asset, since the book version of compiled "Letters to Bill" is sold at party function bookstalls, side by side with the prime minister's collected speeches.

Close observers say the caricatures are a fairly accurate portrait. Whatever the truth, however, they have come to represent the "real" Denis Thatcher in the absence of more substantive information.

Certainly he is his wife's biggest booster, and knows how to play the game when he has to. During electoral campaigns, he is often at her side, shaking hands and chatting up voters, or standing steadfastly in the background, feet firmly planted apart in the stance of the rugby referee he once was. At party rallies he acts as cheerleader -- the first to applaud her punch lines, or shout a hearty, "Hear, hear."

But for the most part, Thatcher has abjured the role of public figure. He has not given a press interview -- except to journalist daughter Carol -- since his wife was elected Conservative leader in 1975. Requests usually are met with the polite insistence that he decided long ago to talk openly about nothing but sports.

The prime minister's office has no official biography of him, explaining that he is a "private citizen." Among the confirmed facts of his life, it is known that he served as an Army major during World War II, and had a brief first marriage that ended in divorce before he wed the current Mrs. Thatcher 36 years ago. He went into the pesticide and paint business started by his grandfather and, when that was sold, became a director of Burmah Oil until his 1975 retirement. He remains active on the boards of a number of companies.

It is generally believed that Denis keeps his mouth shut for fear of putting his foot in, most likely by expressing right-wing views even his wife would shy away from in public.

But to the prime minister, he can do no wrong. "I sometimes have to exercise a bit more tact than he might do," she once admitted to an interviewer. "But if now and then his views differ from mine, so what? They add to the spice of life. I think people adore him, and I do, too."