Pension benefits of present federal and postal workers and retirees almost certainly will be protected for life when and if Congress decides to mandate Social Security coverage for U.S. workers.

Insiders predict that any congressional action to force federal workers under Social Security would contain a "grandfather" clause that would protect incumbent employes and allow them to remain under the more generous civil service staff plan.

Although bills mandating Social Security coverage for federal workers have been introduced, both the Senate and House are expected to await the recommendations of the Universal Social Security Coverage Study. It is due later this year.

President Carter has not advocated mandatory Social Security coverage for civil servants. Aides say he will not make a decision until the study group's findings are in. But the odds are he will go along with the proposal. If so, it is expected to include the grandfather clause for on-the-job civil servants, retirees or their survivors.

The key, insiders say, will be what the administration decides to do in an upcoming revamping of the military retirement system. It is expected to propose changes in benefits and service time, but also to "grandfather in" current service personnel and retirees.

Federal workers have a pension system that is much more generous than Social Security, which is basically a minimum benefits welfare program. Government workers pay more for their benefits, and their pensions are subject to federal tax while Social Security benefits are not.

One change that Congress will seriously consider is the elimination of one of the two annual cost-of-living raises military and government retirees now get. In place of the catch-up-with-inflation boots they get each March 1 and Sept. 1, the Senate and House budget committees have both proposed annual cost-of-living adjustment on July 1 for government retirees. The idea is to put them on par with Social Security recipients who get a single raise each year.

Despite the likelihood of "grandfather" pension protection, federal, postal and retiree groups will continue to oppose mandatory Social Security coverage. They fear any major change could eventually hurt benefits workers already have paid for and are expecting.