President Reagan escaped from his keepers in New Jersey last week, which shows what can happen when the White House staff gets careless and allows him to exercise First Amendment freedoms.

Reagan was at liberty only briefly. He was sighted at a Republican gathering in New Jersey honestly expressing his feelings about the political campaign waged against Supreme Court nominee Robert H. Bork. Before Reagan could be bundled into a waiting limousine and whisked away, he had managed to taunt the Senate's Democratic majority by promising to submit another unconfirmable Supreme Court nominee.

Officials say that Reagan will be punished for his transgression even though he has largely been a model prisoner since Howard H. Baker Jr. became White House warden early this year. They say Reagan will be compelled to serve out the balance of his term in the maximum security compound in the Oval Office and will not be allowed to return to New Jersey until paroled from the presidency. He was permitted to make an Oval Office speech a day after the incident after agreeing to abide by the pre-escape rules of reading what was written for him in the latest White House text.

This is the predicament in which Reagan, at 76, finds himself in the seventh year of his presidency. He is protected from the press, from the public, from congressional critics and from himself. While he is relatively popular and has a useful arms control treaty in sight, Reagan has allowed his inherent passivity and the damage inflicted by the Iran-contra scandal to make him a captive president.

Historians will note that Reagan often has approached the brink of disaster when allowed to speak his mind. As governor of California, he endorsed a "blood bath" to restore order on campus. As president, he once warmed up for a radio speech with a mock announcement of a bombing attack on the Soviets. Reagan's outspokenness last week was reminiscent of a Jan. 26, 1983, appearance in Bedford, Mass., where he startled a friendly group of industrialists by casually endorsing abolition of the corporate income tax "because there isn't really a justification for it."

Throughout his career, Reagan has suffered remarkably little damage from such well-reported incidents. Some of his friends think his forays into frankness have actually benefited Reagan in the long run because they demonstrate that he is more forthright than other politicians.

But presidential self-expression has been effectively suppressed since Baker and White House communications director Thomas C. Griscom arrived on the scene last February. As far as the press or public is concerned, Reagan is now less accessible than during any period of his presidency, including the two years of the oft-maligned keepership of Donald T. Regan.

Reagan has held 40 news conferences as president, 26 in the first term. He held 12 news conferences in two years under Regan and has held only two since Baker arrived. This growing isolation has been partially masked by increased "photo opportunities" which put a carefully scripted Reagan regularly on network news at a minimum of risk.

The president's private accessibility also has diminished. Before Baker and Griscom, he submitted to scores of interviews with newspapers and magazines of general circulation. The White House communications staff rarely trusts him in such forums now and prefers to solicit interviews from friendly journalists.

This isolation strategy has not appeased right-wing critics of Baker & Co., but it has reinforced the impression that the president is slipping. It may have cost Reagan his touch in dealing with anything except friendly fire. Reagan is a performer, who does best with regular practice. He may now be at real risk at a news conference, even though it will be timed to coincide with the announcement of a superpower summit or some other favorable event.

Is Reagan really so unable to take care of himself that news conferences should be semiannual events? Was it so terrible of Reagan to say what he really felt in New Jersey? Is it that damaging to let Reagan be Reagan?

Reaganism of the Week: At the Republican fund-raiser in Whippany, N.J., last Tuesday, the president said: "What's at issue is that we make sure that the process of appointing and confirming judges never again is turned into such a political joke. And if I do have to appoint another one, I'll try to find one that they object to just as much as they did this one."