President Reagan's image makers worked overtime last week after a cancerous tumor was removed from his colon to portray him as unusually cheerful, optimistic and courageous. Their task was made easier by Reagan, who is basically cheerful, optimistic and courageous.

The power of Reagan's personality is the key to his presidency. Here is a man who was shot and joked about it on the operating table and who remains resolute after encountering a slower, more dreaded killer. Here is a man who seems always on stage, but is genuine in moments of personal crisis. Here is a man who enjoys life and expects to complete his second term as president.

Reagan's wit and grace in the aftermath of the assassination attempt early in his presidency endeared him to a nation that did not really know him. His resonant performance endowed him with a mythic quality that became an underestimated ingredient of his political success.

Americans respected Reagan after the shooting because he behaved the way most of us would like to in such circumstances, even though we suspect we might not. "Honey, I forgot to duck," Reagan said to his wife, using the words Jack Dempsey reportedly said to his spouse after Gene Tunney took the heavyweight boxing title away from him in 1926. It was a good line for Dempsey, and a better one for Reagan.

The president is playing an even more important role these days. He is treating cancer as an everyday occurrence, which it is, and trying to demonstrate by the example of his conduct that people can survive and be as good as they were before. The publicity about the operation also has served the useful purpose of encouraging others to undergo neglected medical examinations.

Reagan is known to the world as a conservative, yet he is invariably forward-looking. When he lost the Republican presidential nomination in 1976, Reagan responded with a stirring speech to his supporters that concluded with a quotation from a Dryden ballad he had memorized in boyhood: "Though I am wounded, I am not slain. I shall rise and fight again."

Reagan is as optimistic about America's prospects as he is about his own. On April 28, 1981, a month after he was shot, Reagan addressed Congress with a paean to the first flight of the space shuttle Columbia that Reagan turned into a launching pad for his programs. "We have so much greatness before us," he said. " . . . The space shuttle did more than prove our technological abilities; it raised our expectations once more; it started us dreaming again."

The view that the best is yet to come animates Reagan and makes him curiously youthful at 74. But his aides and perhaps the president himself may be making too much of a good thing by suggesting that he is some sort of superman who will be uprooting trees at his California ranch when lesser men would still be convalescing.

Earlier this year, after I had a minor operation, the president made a considerate telephone call to wish me well. "Remember, the doctor's knife always cuts twice," he said. I had never heard the expression and asked what it meant. Reagan explained that it was a way of saying that it takes more time to recover from an operation than the patient expects.

It is understandable that Reagan, physically active throughout his life, wants to return to work as soon as possible. It is also understandable that his aides wish to demonstrate that the president is fully able to perform the duties of the presidency. But no one can reasonably expect that this will happen overnight.

Nancy Reagan, who grew up in a medical family, wants her husband's schedule kept as clear as possible and is said to be insisting that he should not be rushed to make public appearances. It is a protective and a sensible advocacy, and one that should prevail.

The long-term success of the Reagan presidency requires a healthy and vigorous president. This will be demonstrated by Reagan's performance when he meets Soviet leader Mikhail Gorbachev in Geneva in November, not by the success of August photo opportunities. Reagan's zest for life is admirable, but he deserves some time off. I hope he will take the good advice he gives to others, and not push recovery faster than he should.

Reaganism of the Week: When aides brought a letter to the president to sign to recapture the authority he had delegated to Vice President Bush during his operation, Reagan said, "Gimme a pen."