President Reagan asked reporters at his news conference last week to remember that the words they write are read all around the world and to consider whether the message they send is helpful or destructive to the nation's interests.

Whatever you think of that plea, the fact is that the most important message is the one the president himself conveys by his words and demeanor on public occasions. For the most part, those appearances have been helpful to Reagan in advancing his goals. His wit, his good nature and his rehearsed eloquence stand him in good stead, whether he is delivering a toast at a banquet, a brief political speech or a televised policy address.

But at the last two news conferences, the impression he has created has been one of a man under great strain. The comments on Capitol Hill and in embassies suggest that the tension and anxiety the president displays when answering questions about his policies are beginning to cause concern among those here and abroad who look to the White House for leadership.

That same anxiety is being expressed by members of the White House staff who have come to view each press conference as a hurdle that must be negotiated with care. They have adopted what my colleague Martin Schram accurately describes as a "damage-control" philosophy for dealing with the press conferences: schedule them infrequently, slow down the pace of questioning by lengthy answers and hope that Reagan gets out of them without hurting himself.

That is a defensible, if obviously defensive, strategy. The practical problem is that the president is so strained in executing it--hesitant in manner and nervous in speech--that he undercuts the effort to build confidence in his leadership. The relaxed sense of command and self-control that he communicated so advantageously in his 1980 campaign debates and in almost every formal speech he has made as president turns into a very tentative and tense performance in the press conferences.

Explanations abound. Some say the president's hearing impairment forces him to strain to hear the questions and puts him on edge even before he gives his answers. His aides have tried to reduce this problem by installing an amplifier in his podium.

Others say it is the mental gymnastics of the news conference that the president finds intimidating. He works best when he knows the topic in advance and has his index cards at hand, with the points he wants to make. In the news conferences he held in his eight years as governor of California, the custom was to exhaust one topic before shifting to a new one. He seemed more comfortable with that more structured format.

His critics put forward a much harsher theory: Reagan is under strain because he has such a shaky grasp of the policies for which he is formally responsible that he has a dickens of a time remembering what it is that he is supposed to say about such-and-such a subject.

If that is right, then we are really in trouble--not just this administration but this country and the world. But before accepting that gloomy conclusion, I would like to see how Reagan would do if he were holding a press conference of some kind every week.

He did that when he was governor. But as president, he has held five news conferences in 10 months. On that schedule, every one becomes a very big deal--a big mental hurdle.

The Reagan we have seen at the last couple of news conferences reminds me of the uptight, unhappy Reagan of the Iowa caucus period early in 1980, when his then-manager, John P. Sears, was trying to shield him from the press and public. When Reagan campaigned infrequently, under Sears' constraints, he was a lousy campaigner--always on the defensive. When he was unleashed in New Hampshire, he was terrific.

So it is, I suspect, with the news conferences. People like my colleague Lou Cannon who covered him in California remember those gubernatorial news conferences, not as ordeals to which Reagan submitted, but as opportunities that he exploited easily to carry his message to the people.

Maybe he's lost the knack, now that he is 10 years older. But my guess is that he's just not getting enough practice to feel comfortable in the news conference format. If he had a regular schedule, where on alternating weeks he would have big televised news conferences and small Oval Office interviews with some of the White House regulars, my guess is that he would be better briefed by his staff on a wide range of issues, and much better prepared to discuss them.