THE RETIREMENT of Sen. John Tower (R- Texas), announced on Tuesday, comes as a surprise, not least to those who have already contributed some $1.5 million toward his 1984 campaign. Mr. Tower is 57 years old, at the peak of his productivity by his own reckoning; he faced a difficult but by no means impossible race for re-election. His departure leaves that side of the issues he so strongly championed the weaker, at least in the Senate. As chairman of the Senate Armed Services Committee, he has been a strong supporter of increased defense effort. And he has generally seen his views prevail, even though Armed Services--once monolithic--now has highly competent members of widely varying views on any number of defense and military issues. If Sen. Tower has not been pleased with the general trend of events at home and abroad during most of the 22 years he has served in the Senate, he is entitled to take satisfaction from his successes, and those of his political allies, in changing the direction of many of those trends in the past several years.

When John Tower was first elected to the Senate in 1961, few predicted such a long or successful career. He was considered a political fluke, a Republican who won a special election to replace Lyndon Johnson through a fortuitous combination of circumstances that seemed unlikely to recur. He won the support of many liberal Democrats, for example, on the theory that he would be easier to beat later than would his right-wing Democratic opponent. But Sen. Tower won again in 1966, 1972 and 1978, and has now served as senator longer than all but three other men in Texas history. He has served as a Republican senator longer than any other incumbent today: Strom Thurmond has more years in the Senate, but he did not become a Republican until 1964.

Mr. Tower came to Washington not only as a member of the minority party, but also as a former professor deemed to have little of the political charisma that was considered essential for success in the Senate. That was then. Now it is another story: John Tower has proved to be a shrewd, hard-working and formidably successful senator. So much for the predictions.