SENIOR MEMBER OF CONGRESS may not be offered a $3,000-per year going-away present after all. Thanks to some timely publicity, including stories by Post staff writer Walter Pincus, key Senate and House members are now shying away from the one-shot congressional pension boost that Speaker Thomas P. O'Neill Jr. slipped through the House Two weeks ago without hearings public notice or debate.

According to Common Cause president David Cohen, Speaker O'Neill first raised the idea of a pension plum last spring, when congressional pay went up from $44,600 to $57,000. Under Congress's generous pension plan, a member who leaves after 20 to 32 years gets an annuity of 80 per cent of his average salary for the last three years. Thus a 10-term veteran who retires voluntarily or is defeated in 1978 will get $42,880 a year, based on one year of worl for $44,600 and two years at $57,500. If he stays in Congress till 1980, though, his pension will be based fully on the higher salary and will go up to $46,000 per year.

Speaker O'Neill's idea was to make the higher pension available to retiring members of Congress right away. We fail to see any justification for this plum. A pension of $42,800 is nothing to cry "hardship" about. Moreover, the same gradual, delayed pension increases are going to occur in the executive branch - and will make much more difference to senior civil servants whose salaries were unfairly held down by Congress for years until last spring. Yet Mr. O'Neill has not proposed giving veteran federal workers the same accelerated benefit he thinks his colleagues should enjoy.

Common Cause, which usually blows the whistle on such congressional self-help schemes, has gone along with this one the theory that veteran lawmakers should be encouraged to retire. Well, much as we agree that turnover is generally healthy for the House, we don't believe that years of service, by itself, is any indication of which members may have been there long enough. If the argument for his plum is as strong as Mr. O'Neill and Common Cause would now have us believe, you have to ask yourself why they weren't prepared to make the argument - or even mention the matter - last spring when the pay raise and ethics package was being hashed out.

The best verdict on the whole affair has come, appropriately, from Appropriations Committee chairman George H. Mahon (D-Tex.), the dean of the House. Though Mr. Mahon is retiring next year, the proposed plum would not effect him; after 42 years of genuinely distinguished service, he is entitled to a sneaky passage of the measure was "outrageous," and he's absolutely right.