While politicians have been arguing over the size of the federal government, they have missed the real change -- a more specialized, better-educated and expensive federal workforce.

In the last two decades the number of engineers on the federal payroll has increased by more than 50 percent, to 98,931.

The number of computer specialists has increased by 600 percent, to 46,361.

The number of attorneys has nearly doubled, to 15,532.

And the number of social scientists, psychologists and welfare workers has increased by more than 230 percent, to 58,166.

The executive branch workforce of almost three million (including the postal service) is less and less an army of up-through-the-ranks paper pushers and clerks. Its fundamental mission has changed and the balance has shifted toward well-credentialed and highly paid professionals and technocrats.

This reflects not only changes in society but also the government's shift away from "operations," such as building dams and running its own programs, to a kind of think-tank function.

State and local governments and private contractors and consultants are increasingly the foot soldiers who actually turn federal brainstorms into public reality.

"Today, others do. Federal employes increasingly think, and plan, and analyze and evaluate, and research and dispense funds," said a veteran federal official. "And it's always harder to think up and plan than it is to carry it out."

Ronald Reagan, like Jimmy Carter and others before him, has made a campaign promise to lasso the federal workforce and keep it from growing. But the promise was misleading because the number of people on Uncle Sam's payroll has changed little and has practically nothing to do with the expanding role of government that is the great concern.

During the last two decades, except for a spurt of growth (about 16 percent) in the 1980s, the size of the workforce has been stable. Only its characteristics have changed.

Now, when the public and Congress decree that the government should see that the nation's water (or air, or land, etc.) is cleaned up, for example, it is left to specialists such as Dr. Dean Neptune to figure out how.

Neptune, 36, is a biochemist with a Ph.D. who manages a staff for the Environmental Protection Agency, one of the government's most controversial regulatory bodies. After laboratory scientists, many of them working for private contractors, analyze samples of chemicals found in industrial waste, Neptune and his staff interpret the results.

"We help decide which toxic pollutants [the agency] might consider regulating . . . We are a service group to the rest of the division. Our clients are the project officers who write these regulations."

When the Navy needs to know whether a submarine design will withstand the pressures at a certain depth, civilians such as Dr. Gordon Everstine, 37, develop computer techniques for finding out.

Everstine, a Ph.D. engineer/mathematician with some computer science on the side, works at the David Taylor Naval Ship Research and Development Center in Maryland, where he formulates mathematical models that float onto computer terminals like scenes from Star Wars.

Surrounded as he is with professionals, he said he never sees the kind of bureaucratic problems he reads about in other federal offices. "We just tolerate the bureaucratic aspects" of the job.

Everstine is among the nearly one-third of federal white collar employes (plus two-thirds of the blue collar workers) who are employed in some branch of the military -- a total of one million civilians. The Postal Service accounts for about another one-fourth of federal employes.

The federal engines still are fueled by vast numbers of clerks, boiler plant operators, drawbridge repairmen, forklift operators, cafeteria workers, janitors and other blue collar or general office workers. But their numbers have not kept pace with the growth in higher-skilled jobs and in many cases are declining.

The number of telephone operators, for example, has dropped from 5,000 to 1,000 in just 10 years, even though the use of phones has multiplied, an official said.

Ecologists, soil science specialists, meteorologists, biomedical engineers, radiation control specialists, ultrasound technicians, civil rights analysts, surface mine reclamation specialists, and anti-boycott specialists -- those are among the hot occupational categories these days, according to Paul Katz, an official of the U.S. Office of Personnel Management or OPM, formerly the Civil Service Commission. His office devises standards for jobs and pay.

Some people argue that change has brought lower-skilled workers cleaner, often broader tasks to perform. But it has also apparently created new barriers for them.

"We've created almost a permanent caste system," said Sally Greenberg, and OPM official who specializes in executive development.

"We don't have anymore the mass of low-level operations through which you can rise, and have your talents recognized. Now, there's a gulf . . . People look at those low-level people and say "They may be good at what they do but they're limited. They can't plan. You can't see them in the think-tank setup."

This presents a particular problem for minority groups and women. Though the government has a better record than the private sector in this regard, those groups still are concentrated heavily in lower-paying federal jobs.

Women currently make up a little over one-third of the workforce, or about 4 percent more than 10 years ago (not including the Postal Service). Though they hold almost 70 percent of the lower paying jobs, they have begun to make what Greenberg called "dramatic progress" into supergrade jobs just in the last year.

"They've more than doubled their proportions in the supergrades -- from 2.4 percent to 5.8 percent," she said.

Minority group gains have leveled off after an early period of progress, she added. Only about 6 percent of thetop jobs are held by minority employes. sAbout 21 percent of the federal workforce is minority, compared with 18 percent 10 years ago.

The number of federal executives has roughly doubled since the early '60s, Greenberg said, to 8,000.

Under opposing pressures to reduce Big Government on one hand and to increase public services on the other, government agencies and bureaucratic reformers have adopted the watchwords, "Delegate, delegate, delegate," said Katz.

Accordingly, it is the delegatees who have been proliferating. Over 80 percent of all public employes in the nation now work for state or local governments. There are 13.2 million of them, or about six for every 100 Americans.

And millions more Americans earn federal dollars indirectly, as employes of private contractors or consultants who do everything from running cafeterias to doing medical research to building missiles or rocket ships for the feds.

The EPA is supervising one of the largest public works projects in history - the crusade to revolutionize municipal sewage disposal through improved treatment plants.The government has spent $31 billion since 1970 toward this end. But the work is all being done by local governments and private companies.

Of basic services still carried out at the federal level, most of the routine work is done not by human employes but by computers. "Or we'd have no room for roads. All the space would be filled with government buildings."

Without the silent, humming computers, the Social Security Administration would not be able to send out checks each month to over 35 million citizens who benefit from various federal entitlements. And the Internal Revenue Service would not be able to process 140 billion tax returns and millions of other documents every year.

Until as recently as 10 years ago at some agencies, clerks sat at high desks wearing sleeve protectors and making pen entries into ledgers, some old hands recalled.

While the computers replace thousands of existing routine jobs, and prevented creation of tens of thousands of others, Katz said, "they also created the need for a number of higher-level jobs to design, install and operate them."

Many of those in the lower pay ranks who might have become clerks now work as tape librarians, lower-level computer operators and in other computer support jobs, he said.

Federal workers average nearly one-third more in annual income than the American workforce generally. Officials and union leaders justify the higher pay with the claim that, indeed, the skill level of federal workers is higher and the average education level is higher than that in the workforce at large.

"The average worker in the country just does not equate to the average federal worker," said George Hobt, a pay expert for the largest federal employe union, the American Federation of Government Employees. "We've got more engineers than we've got clerk-typists." There are 69,000 of the latter, 30,000 fewer than the number of engineers.

In fact, according to the government's personnel chief, Alan K. Campbell, the painful political reality is that the price of public servants ought to go up even more, if the govermment is to attract the kind of expertise it needs in a complex society. "I'd argue that we haven't kept up as well as we should in (paying for) the professional and technical advance of government," said Campbell, the director of OPM. "We have to compete for top people with IBM and GE."

The executive branch payroll has actually gone down as a percentage of the total federal budget (from 14.2 in 1960 to 10.8 in 1978), Campbell noted. The more important figure, he said, is the rise in the total federal budget as a percentage of the country's gross national product (GNP) from 14.9 in 1950 to 21.4 in 1978.

"This shows how much of the country's output is tied up in government activity."

Among other things, the ambiguous way in which Congress passes laws, and the increasingly complicated tasks the government is taking on, make this "sort of inflationary spiral in the professionalization of government" unavoidable, says John Tierney of Georgetown University, a specialist in public administration and the federal bureaucracy.

This kind of inflation, like the other kind, gives government more clout. "Having professionals on staff gives an agency a way of legitimizing its decisions -- in other words, a sort of intellectual cover," he said. "Some almost autonomous bureaucracies are untouchable in a way, because of their experts."