The landscape of the country, and maybe the world, was changed by one simple declarative sentence, spoken at Bethesda Naval Hospital by Dr. Steven Rosenberg.

"The president has cancer," he said.

Ronald Reagan had gotten the news in his hospital room shortly before. Whether it was put to him that starkly we do not know.

But everybody who has waited for the results of a biopsy knows that the news, however phrased, produces the sensation of "zero at the bone," as Emily Dickinson described her feelings on seeing a snake.

Rosenberg gave what passes for good news under the circumstances: The chances that the cancer will not recur are between 50 percent and 75 percent.

Nancy Reagan was told before the president. She announced her intention to carry on with her official duties. They both will be engulfed in a tidal wave of sympathy. When it comes to cancer, there are no politics.

Rather extravagant claims issued forth from the president's people about how he sustained Saturday's surgery. He was quoted by his spokesman, Larry Speakes, as saying the day after that he felt "fit as a fiddle." His recovery was described as "spectacular."

Having seen him emerge unscathed from political crises against the odds, the country probably expected that the verdict from the laboratory would be "benign." Reagan, in his presidency, has seemed immune to the vagaries that afflict lesser people.

Although Rosenberg said the president could resume his responsibilities and his daily life as though nothing had happened -- with the single change of periodic and more rigorous checks than followed the discovery 15 months ago of a polyp in the large intestine -- everything is going to be different in Reagan's life.

He will think of himself differently. He will watch himself more carefully. Other people will watch him more closely. He is under intense scrutiny anyway, but now it will be of a different order. Any change in his gait, his voice, his attitude will be front-page news. It will be recorded in stock market tremors, in the value of the dollar, in the demeanor of the politicians with whom he deals. A senator once said, "When God puts his hand on a man's shoulder, I take mine off."

Reagan has a lock on the affections of the American people. They are almost blindly fond of him. He is not exactly a father figure -- he is rather too jaunty and nonchalant for that. He is more a jolly, reassuring uncle who comes to call amid much laughter and many stories. Never mind his views -- wrong, but strong, they are generally considered, and they add to the fondness.

Anyone who criticizes the president when he needs his strength to fight the enemy within will do so at his or her peril. Reagan has won the heart of America, if not its mind. And now, when he needs to be free of aggravation, he is more immune than ever to the give-and-take of political life.

Sen. Robert J. Dole (R-Kan.), his loyal follower, may wish that he had not charged the president last Friday -- the day Reagan went to the hospital -- with "surrendering to the deficit." It was an allowable enough comment at the time, because the president had abandoned the Republicans who had done his bidding and had voted a freeze on Social Security benefits, but it sounds different now.

Dole is a potential presidential candidate, and nowhere will delicacy and reserve be more called for than among that little tribe. The slightest hint or whisper from any of them that the president is falling behind, faltering in any way, could blight hopes forever.

On the other hand, with the anxiety and the sympathy will come a morbid public preoccupation with the president's health. He was overwhelmingly reelected because the people longed for stability and continuity, and they will probably wonder if he will be among the lucky statistics and finish out his term.

At 74, he has a splendid constitution -- "like a man of 40," the doctors enthused after the operation. He has a buoyant temperament. He endeared himself to the public during the tense days after the assassination attempt of March 1981. He responded with quips, he never whined, he entertained no recriminations. Already doctors who did not perform a more complete examination after an earlier polyp was discovered have been under attack. Reagan will, if he runs true to form, dismiss such considerations.

None of it is going to be easy, either for him or the country. There are no precedents at this shattering moment. We never before had a president who we knew had cancer.