Luck is a commodity that can take fortunate caroms into various corners of a nation's life. The President's brush with cancer is good luck for thousands of Americans whose lives will be saved by it. And because of it, the nation will learn that in Ronald Reagan's long run of good luck, the luckiest event was meeting Nancy Davis.

Reagan's brush with cancer will illustrate the fact that the misfortunes of the famous can be fruitful for the nation. The most important thing that ever happened to improve public awareness of, and hence the condition of, mentally retarded Americans was the birth of Rosemary Kennedy. The fact that the President had a retarded sister turned the Kennedy family, and especially Eunice, into a powerful force for demystifying the subject of retardation. Because of President Reagan's experience, millions of Americans will choose to have proper medical examinations, and cancer will be less a subject surrounded by superstitions.

In Neil Simon's "Brighton Beach Memoirs," Eugene, 14, mentions Aunt Blanche: "You see, her husband, Uncle Dan, died six years ago from . . . this thing. They never say the word. They always whisper it. It was (he whispers) cancer! I think they're afraid if they said it out loud, God would say, 'I heard that! You said the dread disease. Just for that, I smite you down with it.' There are some things that grown-ups just won't discuss."

For many cancer patients, the reticence and awkwardness of acquaintances is a burden added to the burden of anxiety. Only through public discussion can people become aware of the encouraging prognoses for many kinds of cancer when detected early. Also, Americans, perhaps with the Salk vaccine as a paradigm, associate public- health enhancements with new technologies. But enormous public-health advances are available, at no government cost, through altered behavior (no smoking, less drinking, more sensible eating, use of seatbelts, non-use of motorcycles, early detection of diseases and conditions such as high blood pressure).

Fortunately for the president, and for a nation fixated by the presidency, the first lady is a doctor's daughter who, if the 25th Amendment provided for transferring power to First Ladies, could have proven, in just eight hours, how formidable a person in a size four dress can be. In George Bush's eight hours as acting president, the deficit increased $200 million. Nancy never would have allowed that.

One of her grandest attributes is her laugh, which is surprising because being a president's wife is no laughing matter. The applause and power go to the president. To the first lady falls the chore of being gracious to thousands of strangers who are more thrilled to be with her than she is to be with them. Furthermore, to be first lady is to be first target of that portion of the carnivorous media that serves one of America's growth industries: the production and distribution of gossip. When the Reagan's leave Washington on Jan. 20, 1989, he -- they, really -- will constitute fully 4 percent of the nation's presidential history. She is a veteran (sportwriters like to modify the noun "veteran" with the adjective "grizzled," but it won't do here) of 20 years in the tumult of politics. Yet it is only recently that journalists, who often are the last to notice, began seeing her as an important part of the Reagan phenomenon.

What makes effective presidents so rare is the fact that presidential power is a function of public affection. The power of the office varies radically -- compare the presidency in July, 1980, and July, 1981. It varies with the grip the occupant of the office has on the public's affection and imagination. Reagan's grip derives, in large measure, from his serene understanding that politics is like baseball, not football.

He conducts his high office the way Earl Weaver conducts his. Weaver, the Aristotle of the Baltimore Orioles, says: "This ain't a football game. We do this every day." Baseball's best teams lose about 65 times a season. It is not a game you can play with your teeth clenched.

Sometime in the 1970s, Americans grew weary of a government with clenched teeth. They have their fill of the "loneliness" and "splendid misery" and other rubbish about the presidency. Reagan is an astonishing political force because he, like his country, has a talent for happiness. He also has help. Indeed, there is an old-fashioned word to describe his not-at-all old-fashioned wife. The word is helpmeet.

To the extent that Ronald Reagan's success as president is related to his reassuring serenity, to that extent Nancy Reagan has been, and will now more than ever be, an unofficial but not unimportant help in governance. The explanation of the president's confidence-inspiring contentment has much to do with the first person he sees in the morning and the last person he sees at night.