THEORIES about why Mario Cuomo stepped out of the presidential race, and what effect his departure will have, are many and varied.One says that the Republicans benefited because the authentic Democratic presidential contenders were kept in the shadows for months while Mr. Cuomo hogged the limelight with his public agonizings. Another analysis says that Cuomo's exit benefits the Democrats since George Bush, not Mario Cuomo, now becomes the issue. All I can think about as I contemplate the colorful New York governor and his recent, headline-holding decision process is a picture I saw of him, again and again, on television: this distinguished politician astride a rocking horse, riding up and down. When I saw that picture and the glee with which Cuomo was offering this view of his whimsical side, I said to myself: He's not running for president. He knew he would be crucified with that picture if he became a candidate for the highest office in the land - Republicans would have used that photo the way they did the one of Michael Dukakis riding in the tank. Cuomo has been a rather frequent guest at Monitor breakfasts. How he can perform. A quip here, some banter there; a humorous jab at the Republicans here, a little self-directed humor there. Over the years one question has persisted: Would he or wouldn't he run for president? How entertainingly he has wrestled with that question. We will miss the show. Or will we? My guess is that the current crop of Democratic candidates is so bland that Cuomo will be kept in the contest - at least theoretically. Talk of a "Cuomo draft" will continue right up to the start of the summer convention - even though the primary results will almost certainly determine who the party nominee will be. Cuomo will feed the idea of a possible draft through the device that has so effectively moved his candidacy forward in the past: Over and over again he will proclaim his disinterest and disinclination. Cuomo is not only a superb politician, he is also a master performer. I've seldom, if ever, found a political leader who so enjoys the performance side of politics. One of his favorite roles is that of Reluctant Candidate. He can play it for laughs. He can play it for pathos. And however he plays it he captivates his audience. Bank on it: He'll keep that Cuomo-draft possibility alive. Some observers have likened Cuomo to Adlai Stevenson. Both struggled over whether to run for president. And Mr. Stevenson, like Cuomo, would have made a great actor. Even as governor of Illinois, Stevenson enjoyed nothing better than spending an evening with friends in which each would read a part from a play, often Shakespeare. But there are important differences between the two. The Adlai Stevenson I got to know as a reporter was perfectly willing to take over the presidency - immediately. He always had a good grasp of where he wanted to take the nation. In his speeches he had carefully detailed his domestic and foreign policy. And the idea of being president and running the country never seemed to awe him. Stevenson, however, simply detested campaigning. He always found the handshaking and baby kissing demeaning - and you could tell it. Stevenson was a poor campaigner in great part because his audiences could readily see that he didn't have his heart in it. The Mario Cuomo I've listened to over breakfasts seems to have misgivings about his readiness to be president. Oh yes, he has been bashing President Bush for letting the country down on domestic matters. But he hasn't shown us that he has any new ideas on how to do a better job. He's short on the "vision thing" too. Stevenson always showed that he had vision. I don't think Cuomo minds the campaigning, even though he is thin-skinned. But I think he still isn't sure whether he has what it takes to be president - and that is what causes him to hesitate.