EVEN IF HE spends all his time on his California ranch riding horses and splitting wood, Ronald Reagan is bound to find his mind wandering to politics from time to time. His staff, which has many good reasons to want him to run again, has spread the word that he will soon be deciding to do just that. His summer offensives, in which he sought with varying success to allay the hostility of many blacks, women and Hispanics, had the patina of a candidacy. Nonetheless, his decision is a personal one and, therefore, unpredictable. As he contemplates it, what do his political prospects look like?

Overall, a little chancier than he might prefer. The president has a large and enthusiastic following. Whatever grumbling you read in the journals and direct-mail solicitations of the Right, the voters in that part of the spectrum are solidly with him. He is the first president since Dwight Eisenhower not to face a primary challenge. Yet he faces an uncomfortably large Democratic bloc itching to vote against him. If it is by no means a majority of the electorate, it is substantial enough that he cannot lose the votes of too many others and still win.

The winning formula for every incumbent president is simple: peace and prosperity. Mr. Reagan can argue that his policies have produced both. But there are a few clouds even on the Santa Barbara horizon. Many voters are uneasy about Central America; they believe that Mr. Reagan wants to do there what President Johnson did in Vietnam, and it is by no means evident that the president can achieve the results he wants without some Vietnam-like escalation. The summer's surging economy and buoyant consumer confidence bodes well for the incumbent. But will the structural deficits that his policies have encouraged spark new inflation or recession between now and the fall of 1984? Not without reason, Reagan strategists these days tend to pause to knock on wood.

For the president's political fate still depends on events over which he does not have full control. He feels strong enough now not to have found it necessary to change course sharply in the summer of his third year in office, as Richard Nixon did in 1971. Yet in the polls he is running only about even with potential Democratic rivals. Mr. Reagan can anticipate the campaign year with a certain hope. But he is not strong enough or clearly enough in command of the political dialogue to look ahead with serene confidence.