OF ALL THE PREPOSTEROUS charges raised recently against Sen. Edward Kennedy, one is more preposterous than the others (although we must concede that Sen. Kennedy has not adequately answered the very serious question as to whether he ever played "post-office" in the sixth grade). For over a year now, a few logic-sadists have contended that Mr. Kennedy's support of former senator Edward Brooke's Democratic challenger, Sen. Paul Tsongas, constitutes conclusive proof that Mr. Kennedy is weak on civil rights.

Please do not laugh. This is a serious charge that was actively circulated among black Democratic voters in Florida by some of President Carter's partisans during that state's October non-binding straw balloting. The Afro-American newspapers have since revealed in an editorial that Mr. Kennedy "dared to support the white Democratic opponent to Republican Sen. Edward Brooke." A little of the sting of that accusation of recklessness is diminished a few sentences later in the same editorial by an acknowledgment that Mr. Kennedy "has the most perfect voting record among whites on behalf of black issues."

We do have a few facts and precedents, in this case, that can probably be stipulated. Taking a walk on your party's nominee, according to the established code of American politics, can be more serious than a misdemeanor. (Just ask John Connally and most of his colleagues from Democrats for Nixon. Party loyalty may not be remembered, but party disloyalty is not forgotten.) At the 1972 Miami Beach convention, Richard Nixon's nomination was seconded, at Mr. Nixon's request, by Sen. Edward Brooke. Presumably Mr. Brooke was then both honoring a political tradition and following the dictates of his own conscience. That November -- in spite of Mr. Brooke's endorsement -- Massachusetts voters, alone of any state, rejected Mr. Nixon and returned Mr. Brooke to the Senate.

Now to the present case against Mr. Kennedy. It is generally agreed that both Mr. Tsongas and Mr. Brooke are intelligent and serious persons. Mr. Tsongas, a Democrat, agrees with Sen. Kennedy on issues like national health insurance. Mr. Brooke, a Republican, disagreed publicly with Mr. Kennedy on these and other issues. Mr. Kennedy endorsed Mr. Tsongas, who won and thereby gave Massachusetts, the nation's most Democratic state, two Democrats in the Senate.

To prove his unyielding commitment to civil rights, some still argue, Mr. Kennedy should have ignored both public philosophy and political values and supported Mr. Brooke, solely on the basis of race. Perhaps we missed something in the argument, but that sort of reasoning in other times and in other places helped to force passage of the Civil Rights Act of 1964 and the Voting Rights Act of 1965 -- both of which Mr. Kennedy sponsored. Maybe next we should require Mr. Kennedy to demonstrate his commitment to his national health proposal by burning his Blue Cross card on the Capitol steps.