GROUCHO MARX once said he would never join any club that would take him as a member. That, perhaps the definitive remark on the meaning of selection, tells something about Jimmy Carter, too.

With the president's popularity descending like Skylab, it's fair to ask not only whether he's on his way out, but how he ever got in. When the newest club member starts breaking up the lounge, one usually seeks to blame the admissions committee. But there is no longer an admissions committee for the presidency of the United States.

There used to be, of course. In the old days, both political parties screened their candidates very carefully. This system had its faults. In fact, the facts were so obvious that the system was abandoned, baby and bathwater both.

All that's left of the parties are hollow shells, quadrennial constructs of the networks. The last gasp of energy most state parties showed was in switching from the caucus system to presidential primaries just ahead of the scythe of reform. The hope was that if party leaders could no longer choose the candidates, at least it was better to turn that task over to the general public than to abandon it to the zealots.

Well, it didn't work. The percentage of registered voters who turn out in presidential primaries is declining almost as fast as the careers of those they select. What this suggests is that the club members do want a screening committee after all. Which isn't very surprising. It's unfair to ask voters to choose from among eight unknowns with no guidance whatever.

But the worst problem with the primaries is not that they don't work very well. It is that they produce -- almost guarantee -- a very special kind of candidate, of whom Jimmy Carter is the perfect example. If most commentators find it hard to put a handle on the Carter administration, it may be because the right handle is so simple it's been overlooked: The president is treating his office as if it were a primary. The flaws he's shown in office are precisely the skills that got him there. He is exhibiting exactly those qualities that win a party nomination now that no one is in charge.

Let's look at some of the more obvious aspects of Carter as president. The most baffling conduct in the Oval Office becomes explainable if we remember how it worked in the most recent primary campaigns. 1. Obsession with detail.

Everyone knows that Carter knows more boring details than anyone else. The subject doesn't matter. He can not only absorb but repeat, verbatim, the most interminable minutiae of every problem, program, treaty and report. Here's a president who's not afraid of press conferences -- it's members of the press who seems to quiver at those meetings because every lever they pull gets three lemons: an avalanche of detail, a clinking stream of everything you ever wanted to know about what you just asked. This deluge may not really answer the question, and it's not perhaps the highest executive skill, but on the campaign trail it's dynamite.

You see, the voters, who are very smart indeed, know very well that it's unfair to ask them to pick a nominee overnight from among eight or nine strangers. They also know one of those strangers is likely to be, president, the person in charge of governing us. So voters set their own priorities -- the first of which is to make sure the candidate is competent.

It would be nice if we had a system where voters could assume that competence, but we don't. So this is what they have to find out for themselves in the dizzying few weeks of a primary campaign, in the 30-second ads and 15-second news clips. Little things like the candidate's general philosophy must take second place until it can first be determined that he just knows his way around. Not right or wrong, not right or left, just competent. Forget about whether he's a great statesman. Does he at least know what he's doing?

The best answer to that question is lots of answers, loaded with detail. This will put the anxious voter at ease. The candidate who stands at the rally in the supermarket parking lot and rattles off the fine print of Salt I, for God's sake, will pass the threshold test. He sounds like someone who's able to do the job. Mastery of detail is not, of course, much of a key to competence. But given the circumstances, it has to do. People aren't given enough time to see how the boat runs -- just whether it will float. 2. Lack of passion.

If it's bad to be incompetent, it's worse to be crazy. So the worst thing a candidate can do on a primary mini-stop is to lose his temper. That suggests instability, of which the consequences in the White House come readily to mind. When James Fallows writes about the Passionless Presidency, he should know that no one with any passion can get through the primaries.

Jimmy Carter never loses his temper in public. Under the most extraordinary provocation, he smiles. He smiles at Brezhnev. He smiles at Mike Wallace. He smiles at Begin. No one can imagine him blowing up (or, by extension, blowing us up).

Lately, he's been a little more testy because his advisers tell him that shows strength. This probably accounts for the "whip his ass" response to his putative challenger. That may have been a mistake. It gave Teddy Kennedy -- who knows about primaries, too -- a chance to smile at Carter. 3. Lack of leadership.

Leadership takes time. To outline something new and then convince people that it's right is a lengthy task. It cannot even be begun in a one-week primary campaign. The opposite of leadership is to tell people what they already know. This tactic is well suited to a primary campaign, and it can be carried off with fair precision, thanks to the confluence of computers and polls.

The candidate can get off the plane in Primary State 12 with a little scrap of paper telling him that in this part of the state, 61 percent of the folks support National Health Care or oppose abortion. This sort of data makes it much easier to answer some questions and to know which others to avoid. You can't be so selective after the nomination -- there are nationally televised debates that the whole country is watching. But on the primary trail you can often get away with merely restating the local mandate. Candidate Carter's campaign actually had a computer aboard to update issues polling data before each local stop.

While the absence of leadership is rewarded, its presence can bring disaster. Any bold plan will have its enemies. If there's time to develop the plan, it may win so many adherents that the enemies won't matter. But there's no time for that in a primary.

Enemies are more dangerous in a primary than in the general campaign. Later, in November, voters must choose between only two candidates. They may not agree with everything the candidate of their own party espouses, but deserting him means deserting a political party, and the only other candidate probably has said things that they don't approve of either. In the primary, though, there are lots of other choices -- and all of the voter's own party. The voter knows so little about any of them that any stand he disagrees with produces an absolute veto -- the others seem better by comparison.

The strict avoidance of any bold leadership was essential to Carter's winning the nomination. Udall stood for some things that some voters didn't like. So did Jackson and Wallace and Harris and Mondale. But not Carter -- at least not as much -- and it worked very well for him. Then.

A corollary of this tactic is the avoidance of expressing broad, general philosophy. To stand for anything large and sweeping only compounds the problems. Carter's carryover of this insight from his campaign to his presidency is what prompted his former speechwriter to conclude that "Carter believes 50 things, but no one thing." 4. Small staff.

To travel far, travel light. It works if you're going to Tibet -- or the White House. There are so many different primaries, each of them unique, that decisions must be made and carried out.very quickly. The candidate with several tiers of advisers just can't respond in time to meet the demands of the qualifying heat. Carter had what was needed -- a small, homogeneous staff whose judgment he trusted. He was wise not to expand his circle during the campaign. He's kept essentially the same circle in Washington, and while some of its members may be first-rate, it's a different ballgame now. 5. Buying off troublemakers.

There are people who care about issues in primaries and who know where each candidate stands. They aren't numerous, but they can cause a lot of trouble. They tend to be articulate. They can poison the air with criticism. You can always placate them on their favorite issues, but this leads to other problems. Jimmy Carter had another solution: He bought them off. To be precise, he promised them jobs. This made a lot of principled people relatively quiet.

These are examples of the most obvious traits of Carter as president. Each is a tactic that was essential to his emergence as the winner of the primaries. There has been no lack of criticism of Carter's performance in office -- as if that were the only point. But it's not. There should be equal concern with the system of selection which assured that his performance would be as it is. Carter's performance is no fluke: It's the result of the way he was chosen.

That system of selection is still in use. We're about to go through it again. Instead of devoting all our energies to castigating Carter, it might be more useful to focus on the root cause of the problem. If we change the script, the casting may improve.

The biggest question in American politics today is not who will run, but how the candidates will be chosen. We can't go back to the old ways of picking them -- we couldn't if we wanted to, and we shouldn't want to. But we can clean up the present act. We can recognize today's primary system for what it is -- anarchy tempered by distrust.

Without reviving bossism or patronage or machines, we might concede that political parties aren't all bad and can perhaps play a role in the selection of their standard-bearers. They are the answer, not the problem. A constituency of party regulars is what's needed: people who have the time and experience to screen aspirants carefully and then make an endorsement.

Party regulars aren't self-appointed. Under the precinct caucus system, they are chosen by their neighbors in a free election. They're representing the rest of us. We might concede that is democracy can be representative, so can politics, that freely chosen representatives of the voters can provide some of the order and information that the present system lacks.

That system is clearly not working well. There are better ways. Why not the best?