The Post reported on Feb. 1 {Federal Page} that 26 opponents of the congressional pay raise will either return the extra income to the Treasury or donate it to charity. The article centered on what the raise in pay meant for each congressman and the appropriateness of voting against the raise but keeping the money.

What The Post, opponents of the pay raise and various "watchdog" groups are failing to realize is that the extra salary members of Congress are going to receive is just the tip of a huge economic iceberg. The real benefits reaped from this pay raise will come from the increase their pensions will receive.

Pensions will rise dramatically because they are based on the average salary of the member's last three years in office. All 535 members of Congress will benefit in this way because it is based on the salary level they were supposed to get -- not on how much they kept.

The pension aspect of all this was virtually ignored throughout the debate before the pay raise was passed, and it has been virtually ignored in the discussions following passage. It is a shame that the most costly aspect of this whole pay increase has barely been mentioned.

As a result of the pay raise, an average House member, retiring under the old civil service retirement system, could see his or her pension increase by about $800,000. This figure is based on a member who is projected to retire in 1994 at the age of 64 with 24 years of service in the House, if we assume a 19-year life expectancy after retirement and a 5 percent COLA on the pension each year. This $800,000 does not include the extra benefits any surviving spouses will receive upon the death of the retired congressman. (Because the Senate passed a different pay bill, those pensions will still increase, but not to the extent of those of members of the House.)

One note to this is that the vast majority of congressional retirees will qualify for Social Security when they reach 62 or 65, and many of them will receive military pensions and/or private sector pensions in addition to their civil service pensions.

Those congressmen who will retire under the new federal employee retirement system, they will not see the sizable pension increase those under civil service retiremenet system will see, but an extremely generous 401(k) plan will more than make up for that.

Congressmen may put up to 10 percent of their salary into a 401(k) plan, and the government will match that contribution up to 5 percent of salary. When invested in one of the approved portfolios, these 401(k) plans will yield each retiring congressman several hundreds of thousands of dollars.

As a retired congressman, I have already received more than $800,000 in pensions, $450,000 of this due to cost of living adjustments. In 1990 I received $90,400 in federal pensions, and this year I will receive $95,000. The real cost of the pay plan is not and never will be the actual increase in salary. It is the additional cost of the pensions that will come back to haunt us.

HASTINGS KEITH Co-chairman, National Committee on Public Employee Pension Systems Washington