Who is Ronald Reagan?

We are now entering the last half of Ronald Reagan's third year in office. As happens in the United States that means that the politicians have focused their attention on next year and its constitutionally inevitable presidential election.

In both parties the fund-raisers are hard at work building up the campaign chests. Every day's mail brings another invitation from some political organization, right, left, or center, inviting us to save the Republic from the scoundrels by mailing in a check at once.

Seven Democrats are compaigning as hard as they possibly can for the difficult and strenuous role of challenger. Two, Walter Mondale and John Glenn, are well ahead of the others. We know quite a lot about these two.

Walter Mondale is a conventional Democrat spending most of his time cultivating the major special groups which make up the basic constituency of the Democratic Party. These are the labor unions (particularly teachers), welfare beneficiaries, blacks, feminists, and Jewish organizations.

John Glenn is an unconventional Democrat, aiming his campaign rather at independents and liberal Republicans. He has given himself distance from the special interest

groups. In the public opinion polls he runs slightly behind Mondale among Democrats but ahead among others. His campaign argument is that he may be less popular among Democrats, but more electable. If elected, he would have fewer debts to pay off.

Americans have a reasonably clear idea of what they would be getting if they sent either of the two leading Democrats to the White House.

Mondale, like Harry Truman, would do his best to advance the interests of the special interest groups which will support him for the nomination and put up much of the campaign funds for the election campaign. He is a traditional politician with a lot of experience in government.

John Glenn's supporters like to picture him as a successor to Dwight Eisenhower - the military hero, above the run-of-the-mill politician, ready to seek out the national interest rather than the selfish special interest, trained to think nationally, not parochially. He belongs to the political center and could speak for liberal Republicans as well as for moderate or right-wing Democrats.

The chances seem to be that after the conventions of next summer we will be watching either Mr. Mondale or Senator Glenn running against Ronald Reagan. True , it is still theoretically possible that Mr. Reagan will not run for a second term. If the present economic revival should turn out to be short lived, Mr. Reagan might well choose to let some younger person take on the job of engineering a second revival.

But if the revival lasts into the spring of next year, and seems then to enjoy a good prospect of continuing - well, have you much doubt about what Mr. Reagan will do?

So it is timely to take stock of what we have learned about Ronald Reagan after two and a half years of his presidency.

Time magazine's perceptive White House watcher, Hugh Sidey, wrote last week that ''everybody interested in the public business seems to have a singular obsession with this fumbling, amiable, enigmatic ex-actor.''

The phenomenon certainly exists. Mr. Reagan's name, face, and activities, past, present, and prospective, dominate newspapers, magazines, and broadcast programs. Interest in what he has done and may do is keen. Speculation about what comes next borders on the obsessive.

It is beyond question that he is amiable. Even his staunchest supporters recognize that when unbriefed by his advisers he fumbles, often embarrassingly. But enigmatic? Is that the right word for what makes him so much more visible than his prospective challengers, and than many earlier presidents?

We know by this time that he brought strong inclinations with him to Washington from Hollywood. We know that if he could he would cut income taxes even more than he has, cut back harder on welfare spending, build even bigger and better guns, shun dealings with the Soviets, and balance the budget.

We know that these inclinations are tempered by his staff in a daily tug-of-war between Reaganites and pragmatists over almost every decision he must make. We know that he does not plan in advance his long-term strategies, but allows the decisions to emerge from that daily encounter which makes the White House a fascinating assignment for a newsman and a source of endless speculation.

Americans are being led by a man who thinks he knows where he wants to go but finds himself forced often by events to slow the pace and sometimes even change direction. So far, a majority of people like it this way. But will they want another four years of it? That probably depends on the state of the economy a year from now.