Who for the vice-presidency?

In Washington these days there is a lot of speculation about whether Ronald Reagan will run again, but what I want to know is who is going to get the Republican nomination for the vice-presidency the next time around?

Most in and around the White House seem to think Mr. Reagan will run again even though he himself may think that four years in Washington is enough after which any man is entitled to get back to a California ranch. Does Mrs. Reagan think the same? Most wives probably would.

But personal inclinations can easily be overridden by arguments from politically persuasive quarters.

It seems to me that the general situation of the Republican Party at Reagan midterm is as follows.

Ronald Reagan has proved to be the most successful Republican campaigner since Dwight D. Eisenhower. Like Ike, he makes people ''feel good.'' He is friendly. He invites friendliness. He turns away what might be a difficult situation with an easy joke.

As President he has continued to be a popular man with a ''nice guy'' image, even though some of his policies are anything but popular. Somehow the public seems to separate Ronald Reagan, the President, from Ronald Reagan, head of the Reagan administration and supposed originator of its policies. It's almost as though Mr. Reagan, the President, lived in one place and Mr. Reagan, the director of policy, lived in another.

There are not two different men living in different places. Washington is not London with the queen at Buckingham Palace and the prime minister at No. 10 Downing Street. In Washington everything is, on the surface, at the White House.

But substance and appearance are separate things. Mr. Reagan is much more a president than a prime minister. He has the final word on policy, but the members of his Cabinet have increasing responsibility for policy.

If, for example, you are an ambassador of a very important country and want to talk serious business with someone in Washington, to whom would you most want to talk? You would make a courtesy call on the President. But when you wanted to get down to details you would go to George Shultz at the State Department and work out with him a program, a plan, or a policy which you both think would be acceptable to your own master and to the President.

The same would apply if you are a big banker. You would go to Donald Regan at the Treasury. If it is guns, then see Caspar Weinberger at Defense. If it's national parks, go see, or picket outside the windows of, James Watt at Interior. The cabinet counts in the Reagan administration far more than is usual in Washington.

All of this helps to explain why the man is more popular than his policies and why he could campaign for reelection in 1984 with remarkable immunity to the dislike which exists for his policies. As things look now any Republican politician would want Mr. Reagan to be leading the party again in 1984. The party has a better chance of winning with Mr. Reagan leading than with anyone else yet politically visible. This, obviously, is one reason Sen. Edward Kennedy chose to sit out 1984.

This, in turn, means that the nearer the Republican Party gets to the 1984 campaign the greater will be the pressure on Mr. Reagan from Republican governors, senators, congressmen, and mayors to run again.

What happens, then, when they come around and beg him for assurance and he tells them that he and his wife think how lovely life would be back on the ranch?

The answer is obvious and easy. They say, just run, win, go back to the White House, stay there as long as you enjoy it, and when you get tired -- resign. But resign in favor of whom?

That is the question that Republican leaders ought to be thinking about right now, and I am sure that many of them are. There are several obvious names. Vice-President Bush heads the list. Senators Howard Baker of Tennessee and Robert Dole of Kansas are two others who have strong constituencies and good public recognition. Among a few the name of George Shultz keeps cropping up as a ''dark horse.''This is a scenario that has not been played out before in Washington. But that gives it the appeal of novelty. It can be done. If Mr. Reagan won the 1984 election with a good successor, and then retired during his second year, the new president would have plenty of time to play himself into the job and go to the polls in 1988 with a good chance of winning.

Besides, that would be the best way of blocking out Senator Kennedy. If you were in Mr. Reagan's shoes, wouldn't you be tempted to play that role?