The United States' security clearance system, which allows 4.3 million Americans access to national secrets but which failed to catch four alleged Navy spies, has offered frightening opportunities to foreign intelligence agencies because security clearance investigators are overworked and many of the checks they make are sketchy, according to elected officials and defense specialists.

Problems with security clearances have grown in recent years as President Reagan's massive military budget increases have meant more work for high-technology defense contractors, and hundreds of thousands of new applications for security clearance from the firms' employes have swamped the investigating agencies, they said.

The Walker spy case, which military officials say has caused "very serious" compromises in naval communications and other sensitive areas, has focused renewed attention on the apparent weakness of the U.S. security clearance program.

Defense Secretary Caspar W. Weinberger last Tuesday ordered an immediate 10 percent reduction in the number of military and civilian personnel handling classified material, and Navy Secretary John F. Lehman Jr. said he wants a 50 percent reduction.

The Defense Investigative Service's 1,500 civilian investigators, who this year are expected to perform 220,000 sensitive clearance investigations of military and defense contractor personnel, plus 900,000 less sensitive ones, are overwhelmed, defense experts and members of Congress say.

These investigators must engage daily in a highly subtle science that, through investigation of applicants' life stories, work histories and financial and emotional states, seeks to make judgments about the potential for disloyalty to the country.

"Loyalty is an ethereal concept psychiatrically," said Steve Pieczenik, a psychiatrist who, as deputy assistant secretary of state for management under presidents Ford and Carter, oversaw the State Department's clearance program. "Investigations don't really point up characterological defects of the individual" that suggest disloyalty. "What constitutes a security problem?"

There is a growing consensus that the current personnel security system is not well-equipped to answer that delicate question.

Many personnel security investigators are considered to be unsophisticated and undertrained, government officials say. Many are low paid, with the starting salary for a GS-5 investigator for the Defense Investigative Service (DIS) at $14,390.

"Well-meaning but politically unsophisticated investigators" must work "without any central guidance or political training, and without any workable criteria of loyalty and subversion," Guenter Lewy, a political science professor at the University of Massachusetts, wrote in a recent book about government security published by the American Enterprise Institute. "One can only hope that some day we will not have to pay a high price for this accumulation of neglect."

Arch Ramsay, then a top official of the Office of Personnel Management (OPM), told Congress in 1980 that clearance investigation is "a very, very demanding job . . . . They are not the kinds of positions that appeal to a lot of prospective candidates."

Pieczenik said many personnel security investigators are "burned out" and are not well-thought of in government.

"They have to be better trained in interviewing, and at weeding out" applicants' character problems, he said. "A good sociopath can counterbalance any field investigator."

Dale Hartig, a DIS spokesman, said criticism of its investigators for being unsophisticated is "completely uninformed." The agency has "one of the finest training programs in the entire U.S. government."

But critics outside DIS say that its "street agents," like the investigators at most security clearance agencies, are overworked. DIS is 16 weeks late in finishing applications for "top secret." The Office of Personnel Management, which checks clearances for a number of federal agencies, is about 30 weeks behind.

DIS' 187 industrial investigators are equally harried checking the nation's 14,000 defense-contractor facilities (3,000 more than four years ago). DIS representatives are supposed to check each facility every six or nine months, depending on its sensitivity. Checking a large high-tech installation can take a team several weeks.

Many of the investigations, especially those for "secret" clearance, are slapdash and almost perfunctory, defense specialists and government officials say. A "secret" clearance basically involves only fingerprints and checks of FBI and Defense Department computer banks.

"The problem is basically economic," DIS Director Thomas J. O'Brien told Congress of the often sketchy investigations for "secret" clearances. "We just don't have the resources."

Sen. William V. Roth Jr. (R-Del.) and Sen. Sam Nunn (D-Ga.), who oversaw Senate hearings in April on security clearance problems, said in a joint statement recently that the investigation for a "secret" classification, held by more than 3 million individuals, is "woefully inadequate." Federal guidelines define "secret" material as that which could do serious damage to national security if revealed.

Individuals handling material that is "top secret" and higher are supposed to be reinvestigated every five years. But the government is about 17 years behind in reinvestigations, officials said.

"They can't take their job very seriously now," Nunn said of personnel security investigators. "It's a complete absurdity. Nobody could possibly handle that number of files. If you were a good, conscientious security checker, you'd probably be already berserk."

OPM, the government's second largest clearance investigator because of its job probing the bearers of atomic and other secrets at the Department of Energy, reports its investigative workload went up 60 percent in the last two years.

O'Brien described the increase in investigations of defense contract facilities as "an explosive growth in workload."

Cuts in DIS staff in the late 1970s corresponded to reductions in defense personnel to be investigated, but added staff in the early 1980s was not enough to keep up with the Reagan-era defense boom, experts said. The agency had 1,740 employes in 1980, but has 2,797 now, according to a DIS document submitted to Congress.

Delays are a way of life in this business, and they are costly. Defense contractors and the military waste almost $1 billion a year in lost productivity because personnel are waiting for clearances, the General Accounting Office estimated in 1982.

It is common for defense contractors, in an effort to avoid wasting employes' time, to request more employe clearances than it needs at any given time, defense specialists said. The companies want to have pools of cleared employes ready to work immediately on a new project, they said. The Defense Department said one-third of cleared contractor employes don't ever actually use the material they're allowed to see.

These unnecessarily cleared individuals further jam up the system, defense specialists said.

Lehman said the Walker case is "an illustration of the fact that the system for granting clearances has become swamped" in various ways.

Critics charge that it is simply too easy to get a security clearance. Sen. Albert Gore Jr. (D-Tenn.) calls it "fast-food security."

"Instead of giving applicants the benefit of the doubt," Fred Asselin, a Senate staff member, testified before a Senate committee, "government should put the burden of proof on them."

From 1980 to 1984, OPM investigated 138,252 persons for clearance, and cleared 138,144, a pass rate of 99.92 percent.

In the military, about one percent of applicants initially are rejected for clearance, while another 4 percent of clearances later are revoked because of drug use, alcoholism or other causes.

Roth added, "I sometimes think it's more difficult to get an American Express card than a security clearance."

A main complaint of defense contractors is the complexity of dealing with the security regulators of more than 20 separate federal agencies, each with its own rules and standards. The paper work is daunting.

"The result is the employe finally throws up his hands and says, 'The heck with the security stuff,' " Lawrence J. Howe, vice president for security at the California-based Science Applications International Corp., told the Senate recently.

Clearance investigators have their own frustrations.

They are frequently hampered in their work because they can't get access to vital records, including state and local criminal files, officials said. The reason is an array of court decisions in the last 15 years, and state and federal laws, restricting government agencies' ability to release private information.

DIS' O'Brien said that such laws make his agency's work "far more difficult and complicated today than ever before . . . . To appoint an individual to a sensitive position involving nuclear weapons or aboard a submarine . . . without knowing his prior arrest records represents a significant security vulnerability."

Roth and Nunn have recommended that Congress require jurisdictions to provide criminal records about applicants for security clearance.

The Walker spy case has prompted a critical reexamination on Capitol Hill and among military and intelligence specialists not only of the Privacy Act of 1974, but also of an entire generation of laws, regulations and court decisions restricting government's ability to investigate individuals, officials said. Some conservatives and military officials said that these changes, which came in the wake of Watergate and revelations in the 1970s about domestic intelligence-gathering abuses, have weakened America's personnel security program.

Roth said recently, "There has been concern even among the moderates and liberals that we've gone too far" in restricting government in personnel security.

Some conservatives are proposing that in order to strengthen the security clearance system, the FBI, congressional committees and police agencies should be allowed to resume collection of intelligence files about subversives -- a practice mostly ended in the mid-1970s.

"We're the only power on earth without files about domestic subversives ," said a Senate staff member knowledgeable about security issues who works for a moderate senator. "You can't operate a great power without it."

David Martin, a retired Senate intelligence expert, said in a recent book about personnel security published by the Heritage Foundation that because of the "near total destruction of the intelligence data base" and other factors, the government is "virtually incapable" of weeding out subversives from sensitive jobs.

Civil libertarians fear a possible backlash against privacy rights in the wake of the spy cases, and say they're against the idea of resuming files on subversives.

"There haven't been any ideological spies in years," said Mark Lynch, a staff attorney for the American Civil Liberties Union in Washington. Instead, spies nowadays are selling secrets for money, Lynch said.

"There's very definitely a danger because of this spy case that ill-advised measures could be adopted," Lynch said. "There's no doubt the right wing is seizing on this." Lynch added that he sees a danger in increased use of polygraphs for military personnel, which Lehman proposed last week.

A staff member for a moderate Democratic congressman said the only solution is for the military to develop a test for recruits that measures a person's loyalty, similar to the military's tests for aptitude in electronics and other fields.

"The word loyalty brings up an image of Sen. Joseph McCarthy," said the staff member. "We're so afraid to use the word loyalty. It's not jingoism. This is a matter of selling out the country."

But Roth said he doesn't want to overreact in the wake of the Walker case.

"This requires a delicate balancing act" between individual rights and the nation's security, he said. "How can we ensure our security and get at information we need without creating some kind of monster, Big Brother?"