Without question, the best preparation for being president of the United States is running for president of the United States. The very qualities required to be a successful president--judgment, leadership, intelligence, the ability to make choices and adjustments, to name a few--are usually required to win the nomination. That a presidential campaign tests candidates' reactions as well as their resilience was obvious once again on the day the world learned that the Soviets had shot down an unarmed commercial airliner.

Like presidents and unlike senators, presidential candidates cannot duck the tough ones by simply being unavailable for comment. As presidents sometimes have to do, presidential candidates must sometimes make judgments before they have all the information they would like to have. They have to trust their instincts, and they are vulnerable to the cruelest of second-guessing, by the press and their opponents.

That was the case Sept. 1, when the candidates were being asked what they would do while a sport-shirted Larry Speakes was telling a Santa Barbara news briefing that the president had no plans to return to Washington. The president later that same day changed his mind. But by then, the six Democrats who seek to replace Ronald Reagan in the White House had already offered their insights and reactions for the nation's electorate, at its leisure, to assess.

One generally observed rule of American politics holds that presidential candidates cannot simply deplore, they must-- possibly as evidence of the kind of strong presidents they would be--include an "action statement," some specific initiative they urge our government to launch. (Intiatives are always "launched.")

The only candidate statements that count are those of that Thursday. By Friday, all the experts and the authors of white papers had weighed in with their wisdom. Thursday's reactions were for the most part the candidates' own instincts. This is the "clip and save" part of the column which you might wish to look at a year from now: the "action statesments "of the respective candidates and where they issued them.

From Ellsworth, Maine, Walter Mondale was heard from while the Western White House was still dismissing any need for abbreviating the president's visit. Mondale joined others in "demanding a full expalanation from the Soviet government." He added: "I believe the United States should take this matter immediately to the U.N. Security Council." Later he expanded his statement to include this: "If I were president today, I would get Mr. Andropov on the phone right now and ask him what happened."

John Glenn was in Fort Dodge, Iowa, where, at noon, he called it "the duty of the United Nations and other world bodies to express the revulsion of the whole civilized world." Glenn added later that "it should be a concerted action by the whole free world, not only by us." Of the Soviets, Glenn said: "They should be censured in whatever way they can."

In Miami, former governor Reubin Askew said: "All the facts are obviously not known, and a full explanation and full inquiry are needed."

Gary Hart, vacationing in New England, said the "administration should consider a range of measured responses, including, for example, cancellation of Soviet landing rights in the United States and the expulsion of Soviet military personnel from the embassy."

In Greenville, S.C., Ernest Hollings said: "I must find out the details." Then Hollings said the United States "must be firm without being bellicose." He made no specific action statement at that time.

In Alaska Sen. Alan Cranston, like his colleagues, condemned the shooting-down of the plane, but also said the incident "is a symbol of the dangers the world faces if we permit this arm race to go on." Cranston offered no specific action statement.

In long campaigns, candidates confront crisis and pressure. By what the candidates say, or what they don't say, at such times, they reveal something of themselves to the public whose votes they seek. Voters are able to get a sense of a candidate's instincts, of the kind of judgment he has and of the kind of president he might be. they reveal something of themselves to the public whose votes they seek. Voters are able to get a sense of a candidate's instincts, of the kind of judgment he has and of the kind of president he might be.