The Washington novel is the pizza of literature.

Just as the cheese-and-tomato pie (an American invention unknown in Italy) tell us nothing about real Italian food, but everything about how Americans perceive it, so the Washington novel reveals little about the real diurnal grind of politics (no one is bidding on the movie rights to the Congressional Record), but an astonishing amount about how we want to regard our leaders.

And judging from the Washington politicians portrayed in the books we like to read, it is remarkable that anyone pays taxes. As much as we may want them to respect our wishes, vote right, and crank out robot letters, which are the verbal equivalent of Cool-Whip, we secretly deplore those traits in fiction, and give our sympathies to Machiavellian depravities only intermittently available in real life.

Of course, verisimilitude is unnecessary for a best seller - or for aspirants to that condition - and "Presidential Emergency" gets along fine without it, perhaps. President Joseph Hamilton is apparently about to defect to Red China during a scheduled trip to Peking, and various persons are drawn into the effort to stop, delay or kill him before he spends the rest of his life playing ping-pong in a Mao suit.

Among them is the central character, Thack Forbes, a rumpled and competent political consultant who once worked for the president until felled by a real-estate scandal. There are also a sensuous female Treasury Department official, a gaggle of espionage types who steal from one another's computers through a system called "Donald Duck," a jealous and wealthy vice president who ran against the president in the primaries, an antique Supreme Court justice, and an old Burma hand (connected to a new American pistol).

Each of these forces deals with the impending defection in its own way, and the ensuing plot - admittedly brisk and at times nearly engrossing - is principally interesting as an illustration of the obligatory rules for how we want to perceive life in the federal city.

Rule One: Certain sophisticated motivations, such as hunger, must be explained in detail. But obvious Washington character traits - venality, envy, narcissism, lust and xenophobia - may be regarded as self-evident. If the scrupulous should inquire after say, the vice president's motivation, it is sufficient to note that his features "abruptly hardened into the vulgar sharpness that characterizes the David portraits of Napoleon."

Rule Two: Murder is an inconsiderable palliative for political problems, in the same color of the moral spectrum as post-dating checks or undertipping waiters.And if Thack Forbes is asked to kill an old and dear friend with "pathogenic accelerator" pills (a CIA nostrum which will do to you in seconds what takes nature more than 70 years), then the reader had better find it plausible. And when Forbes can't do it ("His soul seemed to shrink. He had been unable to commit murder and he despised himself") then the reader had better find that inability surprising.

Rule Three: Unlike other higher primate cultures, Washington rewards the largely unpleasant qualities of ruthlessness, cynicism and cruel toughness above all others. If you can't stand the cold, get out of the igloo. For example, when the president's physician warns him that he will probably die of a stroke if he takes the trip to China, there is only one suitable rejoiner for his chief of staff: "Forget this crap about Walter Reed . . . Hamilton wouldn't go into a hospital now if his toes were falling off. He doesn't cancel something like this because he has trouble getting out of a goddam chair. Now get out of here."

Rule Four: The reader can safely be assumed to imagine what high-pressure Washington looks and feels like, but he still requires he illusion of experience testimony at intervals - no matter how tortured the prose ("The heat wave that had roasted the eastern seaboard since the weekend was on hold . . . Washington would be a caldron again in a few hours") or how trivial the insight ("The food and wine at the Southern White House were excellent, although Hamilton was indifferent to it; he had been advised that a first-rate cuisine impressed visiting dignitaries").

Taken together, these observations suggest that, if we could vote at the bookstores, Caligula would beat Gandhi by a landslide every time. Nor is that surprising: if politicians are the defenders of our sacred rights, then we can expect less macho lunacy of their fictional embodiments than we do from the Magnificent Seven, the Mission Impossible team or the S.W.A.T. boys.

The unhappy aspect of the novel-as-pizza, however, is that our elected leaders get a piece of the pie. If the book-buying public at large rejoices to see politicians as brutal, self-serving manipulators, the same books form the politicians' own self-image. And who is to say how many Bays of Pigs, Watergates and other tough-guy fiascos were instigated because some politicians felt compelled to live up to the kind of image embodied in "Presidential Emergency?" It's food for thought. Curt Suplee