At President Reagan's final campaign stop in Roswell, N.M., four days before the 1982 midterm elections, a small incident on an airport runway demonstrated what many believe is a growing problem of his presidency.

Reagan made a rare decision to shake hands with local supporters along a rope line. As he finished and turned toward Air Force One, the accompanying wire service reporters in the press pool shouted questions at the president about his view of the campaign. Instantly, a line of Secret Service agents closed Reagan off from the questioners. Without saying anything to anyone, the president boarded his plane as smiling senior staff members looked on.

Except for 14 minutes with reporters in the White House Rose Garden last Wednesday, when he claimed the Republican setback at the polls the day before was a victory, Reagan has remained virtually out of sight.

Approaching the midterm of his presidency, Reagan has become one of the most isolated chief executives since World War II. His few public appearances are "photo opportunities" where Reagan can be seen but usually not spoken to or heard. While Reagan's determination to stick with the fundamentals of his economic program and defense buildup is well known, the extent of his participation in decisions to carry out these policies remains unknown.

Trusted senior members of Reagan's staff have shielded him from chance encounters, like the one in Roswell. They have restricted his interviews to occasional, perfunctory affairs, usually with partisans or friendly publications.

Most of the time, access to the president is limited to a handful of staff aides. Reagan sees so few people except in the most controlled situations that some White House aides believe his isolation has begun to reinforce the impression of a president who is distant, uninformed or out of touch.

One of those aides said last week, while praising what he described as a display of Reagan's acumen on economic affairs, "I wish that others in this place could see how much he knows."

In fact, few even inside the administration see much of Reagan at all. On Friday, for example, Reagan's schedule was limited to short meetings with his senior staff and national security advisers, an even briefer meeting with a San Francisco couple who adopted 14 handicapped children, and an appointment with his dentist.

Reagan works hard in his residence, according to aides. But one acknowledged that much of this work is essentially the rehearsing of a trained communicator rather than the intellectual activity of a president who is trying to master difficult subjects.

The president is said to spend much of his time rewriting speeches and polishing points he wants to make rather than acquiring new information.

"He is a performer," one aide said.

The traditional forum in which U.S. presidents perform and demonstrate their knowledge is the presidential press conference. By this measure, Reagan ranks with Richard M. Nixon as among the most isolated of modern presidents.

A comparison of presidents elected since World War II shows that Dwight D. Eisenhower held 50 news conferences between inauguration and the first midterm election, John F. Kennedy held 44 and Jimmy Carter, 39. Lyndon B. Johnson, who succeeded to the presidency after the assassination of Kennedy in 1963, held 44 news conferences from his inauguration in 1965 until the midterm election of 1966.

Nixon and Reagan each held 13 news conferences during the comparable period.

But reporters who have covered both presidents said that Nixon, before Watergate, generally was more accessible for off-the-cuff comments at bill signings and photo events.

Lately, Reagan has appeared determinedly remote, discouraging questions at photo sessions. "I am not going to take any more questions here," he admonished reporters at a picture-taking ceremony two weeks ago to honor New York City Marathon winner Alberto Salazar.

Some Reagan aides find the comparison to Nixon painful, especially those who remember that Reagan held weekly news conferences when he was governor of California.

When Reagan began them in Sacramento, he freely admitted ignorance about many questions but appeared gradually to develop a mastery of press conferences.

During the 1980 presidential campaign Reagan adviser Stuart K. Spencer isolated the candidate for one 10-day stretch after a series of Reagan gaffes in late August and early September.

But he didn't keep him isolated for long, telling Reagan campaign headquarter's officials at one point: "This isn't Richard Nixon. This is Ronald Reagan. It gets a little bit trying at times, for me and everyone else, but we're going to reflect the nature of the candidate."

Unlike Nixon, Reagan is friendly and popular with the press, and his presidential press conferences often appear to have served him well. "They helped keep him sharp and on top of things," according to one aide.

But some close to Reagan said he has been "spooked" by media accounts of his mistakes at press conferences and has lost confidence in dealing with the media.

Yet a number of these same officials said Reagan is far more effective when he holds regular press conferences because they compel him to do difficult homework he otherwise might let slide.

"The president gets rusty if he sees the press only occasionally," said an aide who also observed that infrequent news conferences mean far more ground to cover at each and greater difficulty in anticipating and preparing for questions.

Reagan's isolation is usually blamed on -- or credited to -- deputy chief of staff Michael K. Deaver, his most trusted aide.

Deaver is fiercely protective of Reagan and, with the approval of the president and his wife, Nancy, guards against any scheduling he considers overdemanding.

Others in the administration contended that Reagan's isolation is as much by inertia as design.

"It's not a conscious thing, it just happens," said one official. "I guess it needs positive direction to open it up."

Both communications director David R. Gergen and White House spokesman Larry Speakes have urged that the president have far more frequent encounters with the press.

They have renewed an old recommendation for once-a-month "press availabilities" and an interview of some sort every week, rotated among all media and out-of-town as well as White House reporters.

There are those in and outside the White House who have said that Reagan is older now (he will be 72 in February) and more out of touch and that the problems of the presidency have become too complex for him.

This view is disputed as inaccurate and unfair by close aides who point to decisions made personally by the president on his own initiative, sometimes as a majority of one against the recommendations of his senior staff.

"We have done a helluva job of protecting the president, and that's one of the problems," said one White House official.

"The president makes the best case for his own programs. We don't need to protect him from himself."