Over the past several months, President Carter and the few aides who are privy to his thinking have drastically reduced their availability to the press.

This phenomenon, which was gradual in developing but is now unmistakable, had its beginning in the weeks immediately following last summer's "domestic summit conference" at Camp David and the ensuing Cabinet shakeup.

In the seven months since then, the president has held four news conferences in Washington.

Shortly after the summit, one of Carter's two most important aides, chief of staff Hamilton Jordan, cut off access to him for virtually all reporters.

And it was about this time that the president's other key aide, press secretary Jody Powell, began to shift his own routine by, for example, refusing to grant weekly individual interviews to reporters from Time, Newsweek and U.S. News and World Report, forcing the magazine correspondents into group sessions with him on the grounds he no longer has time to see them individually.

The president took office with a promise that openess and accessibility would be a hallmark of his White House and administration. His own acknowledged abandonment of a promise to hold two news conferences a month is only part of a general trend toward tighter control of information.

In some instances there have been mitigating circumstances. Jordan, for example, has been reluctant to talk to reporters since the beginning of an ongoing investigation into charges that he used cocaine at a New York nightclub. The delicacy of the crises in Iran and Afghanistan has also forced White House officials to be less open in discussing those paramount foreign policy issues.

But the trend has continued, as evidenced recently by Powell, who holds the unique position of total access to Carter along with a mandate to deal with the press.

With the presidential campaign occupying his time and attention, Powell appears to be gradually abandoning the traditional role of the White House press secretary, leaving a vacuum in his absence. In recent weeks, he has spent hours in private strategy meetings and days away from the White House as an advocate and spokesman for the president's reelection campaign.

Last week, for example, Powell was away from the White House for most of three days, spending much of this time in New Hampshire as a spokesman for the Carter campaign.

This week Powell spent Monday and Tuesday in Vermont, working on that state's primary.

Powell's availability to the press is particularly important in light of what John Osborne, the veteran correspondent of the New Republic magazine, has identified as a phenomenon of the Carter White House.

According to Osborne, who has covered the White House full-time since the Nixon administration, no White House in his experience has had so few presidential assistants with access to and knowledge of the thinking and intentions of the chief executive.

"Carter is a more secluded president than Nixon or Ford in some ways," he said. "He shares his knowledge with fewer people. I have the feeling that fewer people know his intentions and his mind than even Nixon or Ford."

Powell, however, is unquestionably one of those privileged few, and Osborne argues the White House press corps in large part has only itself to blame for his growing inaccessibility.

"The press corps is passive these day in a way that would have been unthinkable five years ago," he said. "It's a phenomenon that the press corps has settled for this as passively as it has."

Often neglected as a result of Powell's preoccupation with presidential politics is the daily White House news briefing, a much criticized but long standing White House institution.

These briefings, a regular part of the White House routine under previous administrations, are now never certain to be held from day to day.

In January and February, over a period of 42 normal working days, Powell held 25 of these on-the-record sessions with reporters. On four other days, he participated with other administration officials in "background briefings" in which none of the officials was to be identified by name.

But on the 13 other days -- amounting to more than two weeks out of the two-month period -- there was no White House briefing. Nor were there briefings on the first two work days of March, as Powell campaigned in New England.

In an interview, Powell noted that on many of these days he held informal sessions with reporters in his office, some of them on a "background" basis under which he was not to be identified by name or, in some cases, even as a White House official.

He also argued that these informal meetings frequently produce more news than his on-the-record briefing, a contention with which few White House reporters would differ.

However, as quarrelsome and often useless as the daily briefings are, they remain the only institutionalized, day-to-day system of public accountability by the White House.

As the president's press secretary, Powell is virtually the only member of the White House staff willing to be quoted by name on anything but the most mundane subjects. But even Powell's willingness to speak on the record is more and more confined to the less frequent briefings. Outside that format, he, like others, often insists on ground rules that conceal his name and sometimes his precise place of employment.

Powell has always had a very close relationship with Carter, a source of both strength and weakness. With the president reducing his accessibility, with Jordan a virtual recluse and few others privy to Carter's thinking, Powell, more than ever, is the only man authorized to speak for the White House. In his absence, nothing much is said.