A bill promising federal retirees a cost-of-living adjustment in 1987 should get a boost next week when members of Congress return from their home districts -- where, across the country, nearly 2 million very vocal, very angry retired civil servants live and vote.

The 3.1 percent raise due retirees last month was the first victim of the Gramm-Rudman- Hollings budget deficit reduction act. Congress took away the pension increase, worth on average about $28 a month to the retirees, two weeks after it went into effect.

Because of media focus here, many retirees in the Washington area were aware of the pension take-back. But a number of out-of-towners didn't get the bad news until they opened their January checks and found the anticipated raise was missing.

Two weeks ago Rep. Mary Rose Oakar (D-Ohio) introduced legislation that would guarantee a cost-of-living adjustment for retirees next year, regardless of Gramm-Rudman-Hollings or any other budget action. She had about a dozen cosponsors, including Rep. Stan Parris (R-Va.).

By last Friday, the bill had 80 sponsors. By this time next week, backers hope to have at least 120. It takes 218 votes to get a piece of legislation through the House.

More sponsors are expected to join after Congress returns because of the flak members are getting from retirees in their home towns.

Congressional staffers say that Washington and district offices are getting a number of calls and letters from retirees, and that many members are spending this week meeting with retiree groups.

One House member complained when a newspaper incorrectly listed him as a sponsor of the Oakar bill and asked the paper to run a correction. But he got so many congratulatory calls from retirees, staff members say, that he quickly became a sponsor and then called the newspaper asking that that correction not be run.

Retired government workers are not subject to the Hatch Act, which bars partisan political activity by most government employes. And studies have shown that in both local and national elections senior citizens usually have a higher turnout than most other age groups. Many retirees are active in grass-roots political organizations and have considerable clout in many Sun Belt congressional districts.

California, for example, has 201,000 federal retirees. Florida has 125,000; Texas, New York and Virginia each has more than 100,000. There are 85,000 in Maryland, 88,000 in Pennsylvania and more than 45,000 each in New Jersey, Washington state, Illinois, Georgia, Ohio and Massachusetts, and 52,000 in the District of Columbia.

In addition to the Oakar bill, other legislation is pending to exempt future retiree COLAs from budget-cutting and to restore the 1986 raise. Also, the 500,000-member National Association of Retired Federal Employees has filed suit contending that the Gramm-Rudman-Hollings pension take-back violated the constitutional rights of retirees.