WHEN RONALD REAGAN sits down with his red pencil to start cutting out usuless expenditures in the federal budget, he will find one agency which embodies most of the accusations he has leveled at Washington over the last 25 years.

It is one of those agencies set up with a reasonable idea in mind, but which over the years has grown into something its creators would barely recognize. It exits purely for the benefit of a very small class of favored people who have grown quite accustomed to this special treatment and who would raise an unholy howl if anyone ever tried to withdraw it. It has grown rapidly in recent years, not for any outwardly logical reason but due mostly to the bureaucratic urge to expand. It could be cut back with no great loss to the citizenry or any grave danger to the Republic, but it almost certainly won't be, if only because the principal beneficiary of this little bureaucracy is Ronald Reagan.

The agency is the Secret Service, not the sort of bureaucracy Reagan normally has in mind when he criticizes the federal government. But the critique applies as well to the Secret Service as it does to the Interstate Commerce Commission or the Veterans Administration.

Reagan could make a grand symbolic start to his administration by announcing a reduction in the Secret Service's budget and personnel.

This would demonstrate that Reagan is willing to make sacrifices along with everything else during the austere days ahead. It also would demonstrate Reagan's understanding of one of his own favorite philosophical points -- that some problems (in this case the risk of presdential assassinations) simply can't be solved by throwing government money at them. In fact, Reagan could use this opportunity to take the sort of mundane measures that actually would reduce the danger of his being assassinated -- measures which have been avoided partly because of the illusion of security provided by the Secret Service agents who surround every president and presidential candidate.

The Secret Service, which is an agency of the Treasury Department, was originally set up in 1865 with no thought of protecting presidents. Its primary concern was pursuing counterfeiters. Only in 1901, after the assassination of President McKinley, was it given the additional responsibility of protecting the president. Even this modest expansion of its duties wasn't made permanent until 1951, after two Puerto Rican nationalists tried to shoot President Truman.

The real boom, however, came after the assination of President Kennedy. Some of the new laws had only symbolic value, such as making it a federal crime to attack the president or the vice president. But others greatly expanded the obligations of the Secret Service. In 1965, protection was provided for any former president and his wife, and, in 1968, to widows of former presidents until their remarriage, and to children of former presidents until they reached age 16. Immediately after Robert Kennedy's murder, Congress extended protection to all major presidential and vice presidential candidates.

Besides the responsibility of guarding presidential candidates, the Secret Service has been charged with protecting foreign heads of state and embassies. sThe latter duty incidentally provides a few wealthy Washington neighborhoods an extra layer of police protection, a luxury often mentioned in the local real estate classified ads.

The result of these new duties has been a dizzying rise in the Secret Service's size and budget. In 1963, the agency employed 525 people (plus 170 White House police, who are now called the Secret Service Uniformed Division) and spent about $5a million annually. By 1970, the number of employes had risen to 1,660, and the budget to $27 million. Since then, however, the Secret Service has grown at a rate that would make most bureaucratic empire builders salivate. The total number of employes has more than doubled in the last decade and, even more impressively, the total budget has risen to $159 million -- a nearly sixfold increase in a decade.

No one is in favor of endangering the lives of presidents, their families, former presidents, their families, presidential candidates or foreign embassy personnel. But even when it comes to protecting lives, there are limits to what is reasonable, and the Secret Service has plainly exceeded these limits. w

The agency spends a good deal of time and money protecting people who can hardly be in much greater danger than the average citizen. There is no particular logic to the categories of people guaranteed protection.

It makes no sense to assume that Jacqueline Kennedy was in constant peril until her remarriage to Aristotle Onassis, and utterly secure thereafter. Nor, presumably, will Amy Carter be any less a magnet for psychos the day after she turns 16 than the day before. If thoroughness is the point, one can easily argue that the present coverage is altogether inadequate.

If Chip Carter or Maureen Reagan can be presumed in danger, then why not Jack Ford? If Mamie Eisenhower needs bodyguards, then who is to say Donald Nixon and Eunice Shriver are safe?

In all the years of the great Secret Service boom, there has been no major attempt on the life of any presidential family member, or any former president or his family. On a list of people most likely to get shot at, most of these individuals (one can think of exceptions)would not rank very high.

It is not surprising, for example, that John Lennon was a more tempting target for some lunatic than Rosalynn Carter. Henry Kissinger complained publicly about losing his Secret Serive protection when he left office, and surely he is in at least as much danger as others who continue to get the full Secret Service treatment.

Since most of the people who get Secret Service protection are not evidently at greater risk than many citizens who don't get it, could it be that there is a greater national interest in protecting the families of presidents and former presidents than ordinary people from the violence of our society? I would like to hear a president attempt to justify such a proposition.

It is sometimes argued that presidential family members must be protected because of the danger they could be kidnapped by terrorists wishing to affect national policy. But consider. Would a president pay a greater ransom for his own child than for, say, a busload of orphans chosen at random? If anything, it might be easier for the president to sacrifice his own child for the national interest. A rational terrorist, realizing this, would not choose the president's child to kidnap.

But most terrorists aren't rations, you say, and assassins certainly aren't rational. Of course. That is why trying to predict who might be the target of violence is a hopeless task, why any attempt to limit or cut off someone's protection makes the whole enterprise look futile if not callous, and why the amount spent on Secret Service protection can continue to grow without limit and without much discernible effect.

If the Secret Service were providing nothing more than a couple of heavies to discourage any ambitious young toughs from trying to make history by punching out a former first lady, this might be easier to justify. But all protectees, from the president on down, get the same treatment.

If the president plans to visit Cleveland, for example, the agency will send some agents ahead to scout out his travel route for anything suspicious ("every step," says a spokesman). Then its agents meet with representatives of local and state police units to cooridnate their efforts. Finally, they consider every person in the Cleveland area who has been imprudent enough to make his violent ill will to the president known to the Secret Service.

All this may be sensible for a presidential visit. For anyone else, though, it seems a bit much. You might think that these elaborate precautions would not be taken every time Lady Bird makes a shopping trip to Houston. You would be wrong. Asked about that scenario, Jack Warner, a spokesman for the Secret Service, says firmly, "The process is the same for everyone. Only the number of personnel varies, depending on the notoriety of the person. oWe certainly would screen all the people in Houston who might be a threat. We wouldn't be doing our job otherwise."

How much good all these measures do is also open to doubt. Despite the expanded efforts that followed the two Kennedy assassinations, the Secret Service still couldn't manage to prevent George Wallace's nearly fatal shooting in 1972. The attack on Wallace of course, was the kind that is nearly impossible to prevent, the work of an unknown man waiting quiety in a crowd. The blame for its success lies not with the Secret Service, but with the assailant's shrewd tactics.

Nor could the agency do much to foil the attempt made on Gerald Ford's life by Sara Jame Moore, who fired a pistol at Ford from across a street, luckily missing. (An agent did manage to grab Squeaky Fromme's gun before she could fire it at Ford only a few weeks earlier.)

If an aspiring assassin has the presence of mind to write no threatening letters and to choose his moment and surroundings with some care, the Secret Service is almost helpless to keep him from accomplishing his mision. Perhaps the most revealing evidence on this matter comes from Jack Warner, who admits it i quite possible Lee Harvey Oswald would have succeeded in killing John Kennedy even if current Secret Service procedures had been followed in 1963. Even if presidential candidates had been protected in 1968, there's no reason to think Robert Kennedy would be alive today.

Even when it comes to protecting the president, then -- who obviously is at great risk and whose safety obviously is a national concern -- it ought to be recognized that there are limits. Avoiding assassinations fits perfectly the conservative paradigm of a social goal that is always receding in the distance no matter how much tax money is spent on it.

Nevertheless, every time the system fails, as it inevitably must, there are calls for redoubled effort. And not even conervatives are willing to say that the reduced risk, however speculative, achieved by the added cost, however great, it not worth it.

In fact, most of the dangers faced by current and former presidents, their families and presidential aspirants are of their own making. If a president insists on riding through a city in an open car or on wading into crowds to press the flesh, he leaves himself wide open to attack, almost regardless of what his bodyguards do.

No doubt, presidents and candidates would exercise more caution if they weren't always surrounded by Secret Service agents, who furnish a sense of security that it at least partly illusory. Warner ways that the agency initiates its protection of each candidate by asking him not to ride in open cars or to shake hands in large crowds.

Almost invariably, he admits, the advice is ignored. If they weren't protected by so many agents, candidates and presidents might be less rackless. Simply avoiding these two risks would do more to protect the lives of public figures than aything the Secret Service can do.

So the money spent on Secret Service protection doesn't protect lives so much as it protects egos and traditional habits of campaigning. Pressing the flesh in crowds and riding in open cars hardly amounts to contact with the people in any genuine political sense. In fact, the elaborate vestment of Secret Service protection that descends around a man when he becomes a candidate for president and, if he wins, stays around him until he dies, probably does more than any other presidential perk to create an aura of imperial isolation.

So a suggestion for President Reagan. He should announce that he will insist on a substantial reduction in the number of agents guarding him and his family and ask Congress to cut the Secret Service budget to reflect narrower responsibilities.

He should go further and ask for changes in the law to put Secret Service protection at the discretion of the president or the treasury secretary, so that agents can be assigned to people who are actually in danger. s

He should declare that henceforth he will rely on television, radio and newspapers to communicate with the American people and avoid appearances that expose him to serious risk, preferring to concentrate on running the country instead of repeatedly demonstrating his own manhood.

He should inform the voters that if he is going to ask them to rely less on government solicitude, he is obliged and willing to do likewise himself. These measures would strike an impressive blow against the bureaucratic evils Reagan has made a career of denouncing. They might also make life safer for past, present and future inhabitants of the White House.