Federal and postal groups will spend about a million dollars a week between now and March on ads aimed at convincing Congress that putting civil servants under Social Security would be unwise, unfair and politically unhealthy.

Most government workers shudder at the prospect of what mandatory Social Security coverage would do to their retirement system. As a result, the advertising departments of major magazines, newspapers and TV networks will split $6 million or more for carrying anti-Social Security ads financed by government unions and retiree groups.

The purpose of the ads, which will be targeted at major markets, is to convince 4-million-plus government employes and retirees that their retirement program is in grave danger. Various federal groups plan month-long demonstrations in March to protest plans for mandatory Social Security coverage.

Congress soon will take up a variety of proposals designed to put the financially troubled Social Security system on sounder financial footing. The major bipartisan plan would require that U.S. employes hired in the future pay the 6.7 percent Social Security tax, and probably would put them on a modified civil service retirement program requiring longer service and paying less generous benefits than the current system.

Backers of mandatory Social Security coverage for feds had hoped to head off major union opposition by making the changes prospective. The idea was that current feds would not feel threatened, and would do a minimum of protesting.

But defenders of the current federal retirement program fear--and hope to translate that fear to current employes and retirees--that the shift of Social Security would gut the civil service pension program.

They reason that the civil service program, now funded largely by matching 7 percent contributions from workers and their agencies, would run out of money to pay future benefits if new workers coming into the system paid nothing into it or sharply cut back their contributions because they had to pay into Social Security.

"We'd be sticking out like a sore thumb in a few years," a union lobbyist said, "if they put new people under Social Security. With nobody coming into the civil service retirement system, money to pay benefits would eventually have to be voted each year by Congress."

He said that "given Congress' track record in recent years" on federal retirement benefits "we don't want a system with new workers under Social Security and older workers under CS retirement."

The Fund for Assuring an Independent Retirement (FAIR) is expected to announce the major ad campaign today. FAIR is an umbrella group for 25 sometimes warring federal groups that have decided to bury the hatchet and work together on the Social Security issue.

Unions that got heavily involved in the last election will call in IOUs from members of Congress, asking them to exclude federal employes from any Social Security bail out plan they approve.