A THUMBNAIL DESCRIPTION of a real politician: He was a mama's boy who married, but carried on a long, extramartial affair. His wife may have been a lesbian, although she might have had an affair with a state trooper. As for their children, they had five and they, in turn, had 17 marriages. Two ended in suicide, the rest in divorce. The family is well known to us all. They are the Roosevelts, Franklin and Eleanor of Hyde Park, N.Y. They are not, granted, typical. After all, he was the president.

At the moment, the world is grappling with the news that Eleanor Roosevelt might have had a lesbian relationship. Her inferred lover, Lorena Hickok, was an Associated Press reporter who lived for a time on the Roosevelt estate at Hyde Park. She and Eleanor corresponded, sometimes more than once a day, and the letters are of the passionate variety that puts on the defensive anyone who maintains, as Arthur Schlesinger Jr. does, that this is the way some women talked to each other back then.

No matter. What matters is that none of this -- not the lesbian affair and certainly not Franklin Roosevelt's affair with Lucy Rutherfurd Mercer -- was generally known at the time Roosevelt was president. If Roosevelt complained that nothing -- not even his dog, Fala -- was immune from partisan attack, he must have been speaking tongue in cheek. There was a whole lot about that family we did not know. It turned out that it didn't matter, anyway.

What makes this topical right now is the fact that writers at work are about to tell you things about Edward M. Kennedy you might think you have no right to know. There will be articles about Kennedy's private life, an almost meaningless phrase when it comes to him, and they almost all will be couched in terms designed to make you believe that the more you know about a politician, the better off you are.

Anyway, I for one will read them. I like the theory, and I would like to believe that a person's private life somehow gives you a hint of how he will conduct himself in office. The trouble is, you either never learn enough or things simply do not work out that way. If the saga of the Roosevelts proves anything, it is that there is a difference between public morality and private morality. Say what you will about them, about their lives and their loves, it in no way changes anyone's estimation of the Roosevelt presidency. It turns out that he, for one, may not have been very nice to his wife. This does not change the fact that he instituted Social Security -- that he was a great president.

The lesson applies to Kennedy. No one has had his life more scrutinized than any of the Kennedys. Something like 134 books have been written on the general subject of the Kennedys -- John, Robert, Edward, their parents, their loves and, inevitably, their accidents. At least four books have been written about Chappaquiddick alone, not to mention reams of newspaper copy. Ted Kennedy, after all, is maybe the only senator whose office had to issue a press release denying that he was having an extra-marital affair.

But just what these books and articles prove, is hard to say. They do nothing to clear up the age-old quandary of attempting to reconcile a person's private morality with his public morality. James MacGregor Burns, a Kennedy as well as Roosevelt biographer, says that one type or morality may have nothing to do with the other and he cites, just for the record, Richard Nixon. Here was a man whose private life was "impeccable, but he corrupted the presidency." Burns also offers us J. Edgar Hoover. He was straight-laced in his private life, but he spied on other people's bedrooms.

Burns could have gone on. John Kennedy had his affairs as did, we are told, Dwight Eisenhower. Harry Truman did not, but Roosevelt did and Warren Harding did and then did again -- the last time availing himself of a White House closet. What this tells you of the Kennedy, Eisenhower, Truman and Harding presidencies that you did not already know is beyond me.

This is not to say that a politician's private life is not fair game or that Chappaquiddick, for instance, is not, as John Connally suggests, a fair topic for debate. It is merely to suggest that once the debate is over, you may not know any more about how Kennedy would act as president than you knew before. On that score, we will all have to make up our own minds.

But Connally and others are wrong if they think the American voter doesn't recognize the distinction between public morality and private morality. Back in 1884, when James G. Blaine ran against Grover Cleveland for the presidency, Blaine was accused of a cover-up in government and Cleveland was accused of fathering an illegitimate child. Someone suggested that the solution was to elevate Cleveland to public office and "remand Mr. Blaine to the private station which he is admirably fitted to adorn."

Cleveland won.