Robert F. Diegelman is a little-known Washington bureaucrat who is presiding now over a rare occurrence that could be repeated many times if Ronald Reagan's budget-cutters get their way -- the death of a federal program.

Diegelman is acting director of the remnants of the Law Enforcement Assistance Administration, a much-criticized grant-program that has poured out $7.7 billion to help states and localities fight crime in the last 12 years.

When President Carter sent his fiscal 1981 budget to Congress last January, it included $571 million for a restructured LEAA, an increase from the year before. But the administration, in what LEAA supporters call the "March massacre," drastically cut the program as part of a short-lived effort to balance the budget. LEAA's main component -- its grant programs to the states -- was wiped out completely, from more than $400 million to nothing.

Two weeks ago congressional conferees agreed on a Justice Department appropriations bill that earmarked only $148 million -- $100 million for LEAA's juvenile program and $48 million for a chopped-down research and statistics and administrative role for the rest of what was once nearly a $900-million-a-year agency.

LEAA is still breathing, though. Its authorization is intact. And it still has nearly 500 employes to oversee nearly $1 billion "in the pipeline" already appropriated for more than 20,000 projects around the country.

"If we're going to be closed, we're going to be closed responsibly," Diegelman said.

What happened to LEAA? In recent interviews several officials who played parts in the agency since its beginning in 1968 reviewed the decline of support for the program. Their comments may be instructive to other federal managers whose agencies may soon come under the withering gaze of Reagan budget-cutters.

The consensus seems to be that LEAA's performance in recent years improved markedly, but the agency was never able to escape its image as a wasteful bureaucracy that funded armored cars, night sticks and tear gas, but did nothing to lower the crime rate.

"The program was oversold from the beginning," Diegelman said. "It was naive and simplistic to believe LEAA could have a major impact on the crime problem." At its peak in 1975, LEAA's grant programs accounted for less than 4 percent of the billions of dollars states and localities spent on police forces.

The agency was born in a campaign year when Richard Nixon made crime a major issue. It was a period when America was frightened about riots in major cities, and one of LEAA's first grants was $39 million for riot-control equipment.

After the early concentration on police hardware, LEAA went through several stages of changing priorities, from corrections programs to juvenile delinquency prevention to community grants. "The program became unbelieveably complex," Diegelman said. "The pie was split up so many ways nothing could have real impact."

As the programs proliferated, so did the red tape. LEAA even paid the salaries of the state and local administrators who wrote the applications to get the big money. A whole new industry of criminal justice planners was spawned. There was more than 3,000 of them churning out grant proposals.

When he came to town with Carter, then-attorney general Griffin Bell said a few years ago, he had every intention of abolishing the agency. But he ran into the reality of supporters like Sens. Edward M. Kennedy (D-Mass.) and Strom Thurmond (R-S.C.) on the Judiciary Committee.

So last year the administration sent up a bill to reorganize LEAA instead, cutting the planning requirements and some of the layers of bureaucracy. The measure passed Congress last December and proponents hoped for a revitalized agency with increased funding for innovative programs.

But when the budget crunch came in March, LEAA got clobbered. Charles R. Neill, controller at the Justice Department, said that when the Office of Management and Budget sent out orders to make substantial cuts in departmental budgets, Justice really had nowhere else to look. LEAA was the only grant program at the department.

Joe Mullinix, the OMB official who reviews Justice's budget, said the LEAA program was susceptible because "it was always on the margin" when governors and mayors were asked to rank their needs for federal aid. Tom Madden, former general counsel at LEAA, said such rankings were made with a "stacked deck" because local officials could get most of their law enforcement funds from their tax base and thus ranked federal grants for community development, sewers and highways higher than LEAA.

One of LEAA's most vocal supporters in recent months has been Sen. Howell Heflin (D-Ala.), who was chief justice of the state court system. An LEAA grant helped revamp Alabama's court system, he said. And he listed in the Congressional Record some of the agency's success stories: a career criminal program to emphasize prosecution of longtime felons; funding of "sting" operations; new programs to attack the growing arson problem; uniform sentencing guidelines, and computer assisted systems to track prosecutors' caseloads and make jury selection more efficient.

Heflin's Senate Judiciary subcommittee plans hearings in a few weeks, an aide said, to make a "record" of LEAA's history for the incoming Reagan administration to consider.

Some LEAA proponents cling to hopes the new Reagan at Justice will turn on the funding spigot again. Herb Jones of the National Association of Counties said that Reagan adviser Edwin Meese III told his board last week that he thought LEAA had been a victim of unfair criticism and that the federal government should have a role in the criminal justice system. There's considerable doubt, though, that LEAA will ever again be source of sizable block grants to the states.

Patrick V. Murphy, head of the privately funded Police Foundation, said he's concerned about the demise of LEAA and the failure of this year's presidential candidates to focus on crime as an issue. "In a system as fragmented as ours, there's a need for research. LEAA has been a program well worth the investment," he said.

Jerris Leonard, a Washington lawyer who headed LEAA in the early 1970s and was a criminal justice adviser to Reagan during the campaign, said he too hopes the new administration will take a fresh look at the program. "My view is that LEAA should not be allowed to die," he said. "But you have to get the bureaucracy out of there. It has to be substantially streamlined."

While the philosophical debate continues, Bob Diegelman and his associates go about the business of closing down the dying agency. "This is the first time a major grant program has been cut off and it's important we have somebody around to watch the money in the pipeline," he said. One problem he's already finding is that many of the key accountants and auditors who monitor grantees' compliance with LEAA's requirements are leaving for other jobs.

If the Reagan forces reexamine the LEAA concept, Diegelman said, he thinks they'll concentrate on a few demonstration projects. "It's got to be smaller, more carefully designed. It just can't be as wide open as it was."