Federal and postal workers in the future might have to kick in as much as 12 percent of their salary under a combined Social Security-Civil Service retirement plan being considered in Congress. That would be quite a financial jolt for civil servants who now pay 7 percent of their total annual income for one of the nation's best retirement programs.

Legislation is pending in Congress to force U.S. Government workers, and about half of the nation's state, county and municipal aides to come under social security.

Backers of mandatory social security for feds argue that it is unfair for the government people who administer the program for the rest of the nation to be exempt from it. They say civil service pension benefits, typically twice as good as those for persons drawing social security, are an elitist concept that give ex-government aides much better retirement incomes than the average American who is forced to pay into social security.

They also say government workers can retire much ealier and that many of them then take jobs in private industry allowing them to also earn benefits under social security.

Federal and postal workers see it differently. They admit their program is better, but argue that they pay for the difference and also pay taxes on retirement benefits from which social security recipients are exempt.

If merger comes - and that seems to be in the cards at some future date - federal officials say the upshot might be that federal and postal employes would be under mandatory social security, but also be able to buy additional pension benefits. The price would be high.

A similar system is already in operation for the Tennessee Valley Authority. The quasi-federal agency has about 23,000 employes who now pay into both social security and the federal retirement program. The TVA system is considered a likely model for the rest of the federal establishment.

TVA employes now pay 6.13 percent of income (up to a maximum of $22,900) into social security. In addition, they can buy into the government pension program by paying an additional 6 percent of annual salary into the fund. (TVA pays 11.5 percent of payroll to match it).

By contrast, federal employes and their agencies each pay 7 percent into the civil service retirement fund.

Under the TVA system, employes who retire get benefits under both social security and the federal retirement program. Since early retirement is possible in government, some TVA retirees draw only their federal pensions until they reach the mid-60s when they also qualify for social security.

It is a costly program to employes of the TVA.But it does provide excellent retirement benefits. Their social security payments are not taxed.

TVA employes have the option of paying less, only 3 percent, into the federal retirement program. Naturally this reduces their retirement incomes.

TVA retirees get annual cost-of-living adjustments. Federal and military retirees now get adjustments every six months to match increases in the cost-of-living.

Under the TVA system, however, pension raises are for a maximum of 5 percent. If the cost of living is higher (currently the inflation rate is 12 percent a year) they still get only a 5 percent adjustment each January.

It will be December, at the earliest, before the Universal Social Security Coverage study group completes its final report. But, as pointed out here Sunday, indications are that it will recommend mandatory social security coverage for federal workers. If it does, odds are good that the government will take the TVA system as a model for revising its current retirement program.