Will he, won't he?

All the indicators that the experts can think up point to the obvious conclusion - that Ronald Reagan will run for reelection to the presidency of the United States.

But I would like to go on record as suggesting that ''all the indicators'' are not necessarily decisive. I for one am not going to be surprised if the indicators prove mistaken and Mr. Reagan, at the very last possible moment, decides after all that life on the ranch in California can have more charm than another four years in the White House.

One factor that would certainly influence me against running, were I in his shoes, is that now, nearing the end of his third year in the White House, he is well ahead of the game.

On balance, it has been a good run for him and for his policies and purposes. He wanted above all to cut taxes, rein in on the steady rise of welfare costs, and rein out on defense.

Those things are done, and carried about as far as they can go.

Taxes cannot be cut further. There will be higher, not lower, taxes during the next four years, regardless of who the president is.

Welfare has been restrained as much as is politically possible. Another term would not give Mr. Reagan, or anyone else, the political ability to reduce further the levels or the range of the ''entitlement programs.'' Over the next four years whoever is president will be concerned primarily with finding more revenue to sustain continuing programs.

Defense has been unleashed to the limit of ability of the defense contractors. They are already overstretched trying to fulfill existing contracts. And the programs themselves have been stretched to the limit of political tolerance. The defense task during the next four years will be to try to complete the programs already launched and then find the people to man the new weapons.

To a considerable extent, Mr. Reagan has worked himself out of his job. Not only has he achieved his main purposes, he has in the process presided over a remarkable improvement in the American economy. Inflation has been brought almost to a standstill. The economy is enjoying a considerable recovery, likely to survive into 1984.

Mr. Reagan could step aside now with a feeling of having achieved his main purposes and with the country feeling considerably better about the state of its affairs than it did when he took office. He could retire with considerable applause (blacks, organized labor, unemployed, and those cut from the welfare rolls excepted).

Then give a thought to what is likely to happen if he runs, wins, and spends the next four years in the White House.

The economic recovery, which is the happiest event of his first term, is limited. It is not a full-fledged recovery affecting the whole economic spectrum. It is held down in middle gear by a shortage of capital at low interest rates. Long-term capital goods are not sharing in the recovery.

If the great majority of economists know what they are talking about, the man who is president over the next four years must find a way to reduce the deficit by a combination of reduced spending and revived taxes, or the recovery is likely to waste away and never attain that full speed ahead which is the goal.

Foreign policy will not be easy in the years just ahead. Mr. Reagan can attain a quick win in tiny Grenada - as he has. But there is no quick win in sight in either Central America or the Middle East. We are further from peace and stability in those areas than when he came to office. And relations with the only other major power in the world, the Soviet Union, could hardly be worse.

The last American President to enjoy a happy reelection and a reasonably successful second term was Dwight D. Eisenhower.

Lyndon Johnson won a great victory in 1964 and bowed out in 1968, probably wishing that he hadn't won in '64. His country had turned against him over the Vietnam war. Richard Nixon's second, unfinished term was a disaster.

Mr. Reagan looks like a much more successful President right now than he has any reasonable expectation of looking if he wins a second term. He can probably have it if he wants it. But to take it is to ask for four years of new problems in both domestic and foreign affairs, for which he is ill-equipped by temperament and inclination. He has already had during his first three years most of the triumphs he could hope to win in eight.