Look for a major congressional fight to save an estimated 8 percent cost-of-living raise due U.S. retirees this September from budget cutters.

Both the Senate and House are moving toward approval of legislation that would restrict federal, postal and military retirees to a single COL raise each year. Those retirees by law not get inflation-catchups every six months, in March and September.

The Carter administration has asked that retirees be limited to a single COL boost each March. The Senate Budget Committee wants the COL raise for government-military retirees made each July. Either way, the boost due retirees this September -- estimated to be worth at least 8 percent -- could go down the tubes unless Congress agrees to let it happen, and begin the new one-shot COL system in calendar 1981.

Senate and House conferees are meeting now on different budget resolutions they have already approved in principle. The biggest cost-cutting item affecting U.S. retirees is the proposal that would limit them to one COL a year.

Insiders believe the change from two COL raises to one a year is inevitable. The question, they say, is which date will be approved for the single COL adjustment -- either March or July, and whether Congress will let retirees get the COL already due them for September.

Halfway through the countdown period, the consumer price index that controls the amount of the COL increases, shows that a raise of at least 4.3 percent already is due retirees. By the time the raise actually is put in effect in September, it is expected to be worth at least 8 percent.

Federal, postal and retiree groups are planning a well-financed campaign to convince the Senate and House to live up -- one last time -- to their promise that retirees get two raises a year. They expect Congress to adopt the single COL raise, but will argue that retirees should get the final installment of their 1980 COL raise in September.

If they lose out, that is if Congress decides to put the single COL raise into effect immediately, retirees could have to wait until March 1981 (under the White House plan) or September 1981 (under the Senate plan) for their next such raise.