President Carter's federal hiring freeze, allowing agencies to fill only one of every two vacancies as they occur, will reduce employment by 19,500 jobs this year -- if:

The freeze remains for at least 90 days, and agencies are kept from going on a hiring binge whenever the freeze is finally lifted. (Experience shows that government hiring freezes tend to self-destruct within a fairly short-time.)

Special interest groups, from poor people and coal miners to fat cats flying private jets, can be prevented from circumventing the freeze. They will try to have their pet projects, programs and personnel removed from the job-cutting while others do the economizing.

Federal officials don't thumb their noses at the hiring freeze, and outflank it by hiring hundreds or thousands of contract personnel (who don't show up in federal job totals) to do work.

The idea behind a freeze is to reduce the work force painlessly via attrition because X-number of federal workers quit, retire or die each month. And, if they are not replaced person-for-person, the government will get smaller via attrition. It works. Up to a point.

It is a fact that hiring freezes tend to frighten people in government, causing many who would normally retire or seek other jobs to stay put. In other words, the first thing that an action designed to cut the work force by attrition does is to cut the rate of attrition.

Federal hiring freezes are also easily stricken by a Washington disease called "you're-going-to-cut-my-what?" That means everybody is for economy, so long as it is practiced with discretion, with somebody else. Here's a classic case:

Back in the good old days when we had the Vietnam war and the war on poverty, Congress slapped a tough freeze on government hiring. It was so tough it appeared the government would consume itself, and soon be staffed by no more than a handful of aging clerks and administrators.

After congratulating itself, Congress took a look at what it had wrought. My God! Congress said, we can't freeze the Defense Department. Defense then employed 54 of every 100 civilian federal workers. So in one fell swoop (or swell foop) Congress exempted more than half the government from its freeze.

Next, Congress exempted the second largest federal agency, the Post Office Department (now U.S. Postal Service) because, gosh, people had to get mail.

Quickly, thereafter, Congress exempted the third largest federal agency, the Veterans Administration, from the freeze -- can't cut benefits to veterans during a war.

Within minutes of the exemptions for Defense, Post Office and the VA, somebody reminded the members of Congress that they do a lot of flying. vPresto! An exemption for the Federal Aviation Administration and its air traffic controllers. Then a thaw for the good old Internal Revenue Service.

Then it was pointed out that Social Security was doing lots of nice things for people, as was the Agriculture Department.

You get the picture. Within a very short time the tough freeze applied to only a handful of federal operations -- like the State Department, Tariff Commission, Subversive Activities Control Board and a few other unloved outfits.

Since then Congress and presidents have grown a little smarter -- or more realistic. That is why this freeze isn't total, and why it won't bring the bureaucracy to a grinding halt or even stop hiring. It will allow agencies to continue to hire people, but on a one-hired-for-every-two departures basis. Insiders expect it to remain in force until midsummer.

Tomorrow: Promotions and the freeze: Get your raise now or kiss it goodbye for at least a couple of months.