THE ONLY BLACK in the Senate - that is the way Sen. Edward Brooke had come repeatedly to be described in the past few weeks as it became plain that his campaign for re-election was in trouble. The other prevailing description had only to do with the hot water he was in - up to here - concerning his financial conduct and misconduct. Both thumbnail sketches were true. And yet taken either individually or together they did not begin to add up to an adequate appraisal of the man who was defeated on Tuesday in his attempt to be re-elected as Republican senator from Massachusetts.

For now we will leave the weighing of the evidence on Mr. Brooke's alleged improprieties to those charged with investigating and acting on them. You don't have to be soft on ethical slips and slides or even plummets to observe that over the years Sen. Brooke's voice was an important and wholesome influence in the Senate. Yes, as the drama-minded keep insisting, he was the only black in the Senate; but to say that is also in no way to begin to define his role there as a black . Sen. Brooke has been unyielding in his pressure for civil-rights advances of a very sturdy, conventional, even old-fashioned kind. He distinguished himself in this connection by his work on housing programs on the Banking Committee and his impassioned pleas for his colleagues not to turn the clock back on their own progress toward desegregating the nation's schools and other institutions. He was totally - immoderately - committed to pushing ahead for moderate solutions, for unrelenting progression of racial gains leading eventually to his ideal of the integrated community.

The same cast of mind and temperament animated Sen. Brooke's involvement in national-security affairs, in which he played an important role. We think of his involvement in the mind-breaking arguments over the U.S.-Soviet strategic relationship as an example. Sen. Brooke, a rarity among his collegues, troubled to read himself extensively into the weapons and strategy literature, and his role in helping to educate his fellow senators and to press a succession of administrations toward some wise decisions was notable - never mind that on some the really big ones (such as a MIRV flight-testing ban, which he proposed when it might have mattered) he lost.

His has been, in other words, a cautious, undoctrinaire cast of mind, independent, dogged and unwilling to be dazzled, overwhelmed or rushed. It is to take nothing away from his successful opponent, Paul Tsongas, to observe that Sen. Brooke's achievements were solid and that his presence will be missed. He was a good senator.