Federal and military retirees, including 100,000 in the Washington area, are one step closer to losing a part of the program that guarantees them the best inflation insurance in the nation, full cost-of-living catch-up raises every six months.

The Senate has endorsed recommendations of its Budget Committee that would drop one of the COL raises that retired government workers and military personnel now get each March and September. The Reagan administration says limiting retirees to one COL raise every 12 months will save the government almost a billion dollars a year. Senate approval of the proposed spending cuts means that the Governmental Affairs Committee must formally approve the COL payment change, unless it can come up with another plan that saves the same amount of money.

Earlier this week the House Budget Committee also backed the Reagan administration proposal to give retirees inflation raises every 12 months, instead of twice a year. If the House approves that package -- which contains many other spending cuts -- the Post Office-Civil Service Committee will be required to approve it unless it can come up with a compromise that reduces federal pension outlay costs by the same amount. The Senate and House Armed Services Committees must, under congressional budget rules, do the same thing for military retirees.

Many retirees are confused by the congressional budget-cutting action. Some still do not know, or do not believe, that their twice-yearly inflation raises are in danger. This is why: Newspapers have reported, correctly, that the Senate voted down an administration request to cut COL benefits. It did, but that is another issue. The Senate has agreed not to change the SIZE of the inflation adjustments. But it has okayed a cutback in the FREQUENCY of the raises. The National Association of Retired Federal Employees, biggest of the federal groups, says many of its members are still unaware of the threat to their two COL raises. Meantime Congress is moving rapidly to limit federal and military retirees to the same one-a-year raise that people get under Social Security.