The reason Kennedy did it

Senator Kennedy is a superb politician. More than that, he's what is sometimes called a political animal. That is, he possesses a particularly good feel for how to take actions in public life that will be favorable to himself.

Thus, this conclusion is inescapable: The real reason Kennedy decided to defer his presidential bid was because he sensed the likelihood that he would be defeated if he ran in 1984.

Of course, personal reasons entered into his decision. He's a good father and his children's persuasion doubtless played an important part in his stepping out of the 1984 presidential contention.

But before the children made their plea it is understood that Kennedy was already leaning toward not running. Both his personal political antenna and his political advisers were telling him:

1. President Reagan was set to run again, and this would mean that the Republicans would have a very formidable, perhaps unbeatable, candidate - unless the economy should be in bad shape.

2. The economy would doubtless at least be improving by the fall of 1984 - and this would almost assure another four years for the incumbent President.

At the same time that Kennedy was coming to this decision another superb politician - the President - was himself coming to a parallel conclusion: Kennedy was beatable.

In fact, people close to the President were saying that Kennedy was Reagan's ''favorite'' opponent for 1984. By this, the President was understood to mean not only that Kennedy was beatable but that Kennedy was perhaps the most beatable among the current crop of possible Democratic challengers.

The Reagan view of Kennedy is said to be along these lines:

He thinks that the Massachusetts senator is immensely personable as a campaigner and quite able as a legislator. In no way does he underrate the Kennedy charm. And he sees in Kennedy a man whose speaking ability before a live audience may be unmatched by those of any other politician on the scene today - including himself.

But Reagan is said to give Kennedy less than the highest grade for performing on TV. He sees the same loud, strongly assertive Kennedy manner that goes over so well with a real audience as being too ''hot'' for television.

Reagan is said instinctively to like Kennedy, much as he differs with him philosophically. He has not had occasion to develop a personal relationship with the Massachusetts senator. But he relates well with Kennedy's bantering good humor.

In Kennedy the President saw a possible opponent who would be offering the voters the political philosophy completely opposite of his own - and who would be locked into that position. And this is the basic reason he would have liked to run against him.

Reagan feels the public mood still is very much along conservative lines. He sees nothing in this last election to convince him to the contrary. Therefore he liked the idea of the clearly conservative Reagan running against the clearly liberal Kennedy. And he thought with Kennedy as his opponent he would have the ideological edge with a majority of the voters.

Well, quite clearly, so did Kennedy. Thus his decision not to run.

Kennedy's informants are telling him more than just the news about the likely upturn in the economy. They are saying that while there may be signs of some turning away from the conservative mood of 1980, there is still a dominant thread of public opinion that favors less government and less government spending.

They are saying that, while Kennedy may not be the locked-in liberal that some of his critics say he is, he still would have a very difficult job to reshape his political thrust in a more conservative - and hence more acceptable - direction without appearing oppor-tunistic.

There were other reasons for Kennedy not running at this time. His imminent divorce isn't likely to help him among some of his older Roman Catholic supporters. And he obviously knows that he would have to face some kind of a media rerun of Chappaquiddick.

In any event, Kennedy is out of it. Or so it seems. Could he be dragged back in? Only if the economy moves ever downward and there is a public outcry for Kennedy and a return to the liberal approach to mending the nation's ills.