Even if they barely squeaked to victory on Election Day, incumbent members of Congress who hung on will cash in in a big way when they retire or lose their seats, thanks to the effect of federal pension rules on the 25 percent pay raise the politicians will get in January.

Rank-and-file federal workers are due to receive increases of 4.1 percent next year. Federal and military retirees and Social Security recipients will get 5.4 percent cost-of-living adjustments in January.

Federal retirement benefits are based on length of service and the retiree's highest three-year average salary.

In January, pay for members of Congress jumps to $125,100 a year. Both House members, who are elected for two-year terms, and senators, who are elected for six years, will benefit under the salary averaging rule.

The National Taxpayers Union estimates that veteran House members who serve another two years will, thanks to the averaging formula, get an extra $400,000 in pension benefits. Some will get an additional $2 million because of the 1991 raise, the organization says.

Members of the Senior Executive Service, whose pay is linked to congressional raises, also will benefit, even if they work only a few days after their proposed raises take effect. The SES increase won't be official until President Bush issues an executive order, which he is expected to do next month.

The raises would give a big break to SES members, even if they delay departure from December to January to cash in unused annual leave. Annual leave is figured at the current salary rate, not what it was worth when it was earned. Under the proposed pay raises, SES pay -- which now ranges from $87,000 to $108,300 -- would go next year to $101,300 to $138,900.

The increases would increase instantly the value of each SES member's annual leave by between 13 percent and 25 percent, depending on grade level. What a difference a day makes! Virginia Insurance Session

Health insurance expert Gordon Brown is scheduled to speak Friday at a luncheon and insurance seminar sponsored by the Virginia Federation of the National Association of Retired Federal Employees. The meeting is scheduled for 11:30 a.m. at the Best Western Motel in Winchester, Va. For details, call Anna Argenbright at 703-662-6730. Brown is author of the "Open Season Guide," which is aimed at helping workers and retirees pick the best insurance plan during the hunting season, which ends Dec. 10. $500 Penalty

That's what workers and many retirees could face in 1991 if they fail to get prior approval for non-emergency hospitalization. Retirees with Medicare Part A (hospital insurance) and people hospitalized outside the United States don't need advance clearance. But others would face a $500 penalty.

Insurance companies are planning to set up toll-free telephone numbers for calls seeking clearance. It should be sought at least seven days before the hospitalization. The clearance requirement is new to the federal health program, but it has been standard in private health plans for years.