Believing Mr. Haig

Secretary of State Alexander Haig has got a credibility problem.

I was in a group watching the news the other evening. He came on the screen at a congressional hearing. He was being assertive about events in and affecting El Salvador. He pounded the table. He declared in passionate tones that the rebels in El Salvador are being directed, trained and supported from Nicaragua and Cuba. He said he had ''overwhelming, irrefutable and unchallengeable'' evidence.

Well. The tone and choice of words left the audience less than totally convinced.

Partly it was because of choice of words.

After all, no proposition is unchallengeable. And in politics few propositions are overwhelming. The evidence, if it were massive enough, might be irrefutable. But Mr. Haig declined to provide the evidence. There is something shaky about a position which is said to be based on irrefutable evidence unless the evidence is placed on the table.

But a couple of days later it was arranged to have the CIA give a briefing to the members of the Senate and House Intelligence Committees on outside aid to the Salvadoran insurgents. The chairman of the House Committee is a Democrat, Edward P. Boland of Massachusetts. After hearing the evidence he committed himself. His formal statement read as follows:

''The insurgents are well-trained, well-equipped with modern weapons and supplies, and rely on the use of sites in Nicaragua for command and control and for logistical support. The intelligence supporting these judgments provided to the committee is convincing.

''There is further evidence that the Sandinista Government of Nicaragua is helping train insurgents and is transferring arms and financial support from and through Nicaragua to the insurgents. They are further providing the insurgents bases of operation in Nicaragua. Cuban involvement -- especially in providing arms -- is also evident.''

That way of putting it is persuasive. It is specific. It is carefully phrased. Now we know what we are talking about. Salvadoran insurgents go to Nicaragua where they get training probably both from Sandinistas and from Cubans. When trained they are sent back to El Salvador where they carry out guerrilla operations probably under radio direction from a command and control base in Nicaragua.

There is no reasonable doubt in my mind that the weapons they pick up in Nicaragua or are issued on return to El Salvador have sometimes come from Cuba, although there are other sources. Some are captured from Salvadoran government forces, and some, it is said, are actually bought from those same government sources.

But, there is nothing really new about the above. It has been reported by various American and other newsmen long before Secretary Haig made his assertion. It repeats a familiar Central American pattern.

From the end of the Spanish Empire in the Americas to the present, insurgencies in Latin America have been like this. They are usually organized, mounted, and supported from outside.

Many a ''filibustering'' expedition (what they used to be called in Teddy Roosevelt's day) has been mounted from Florida. But if one side in any Central American civil war is aided from Florida (as in the Bay of Pigs case) the other side has to look elsewhere for aid and comfort.

There is certainly nothing surprising in anything Mr. Boland of Massachusetts reported after listening to the CIA's dossier of evidence, and there is no difficulty in believing him. When stated in plain, realistic terms, the case is persuasive.

But then we come down to the real question. So what?

This is what we all should be considering and debating. Mr. Haig's declaration was intended to persuade Congress to put up more money for more US arms to the ruling junta in El Salvador. Perhaps Congress should. But stepping up US aid to one side tends to stimulate more and similar aid from other places to the other side. Are there other options?

Yes. You could enter into negotiations with Cuba and Nicaragua and attempt to arrive at a nonintervention formula. Or you could blockade the coast of El Salvador. Or you could try to get an internationally supervised election in El Salvador.

Would it not be well to think through all possibilities for action before going ahead with a competition in seeing which side can pump in the most guns to either side in El Salvador? Guns may be the final answer, but the case has not yet been aired sufficiently in a public debate. As yet there is no perceived national consensus. In fact the polls show the public in a highly skeptical mood. Mr. Haig is not yet sufficiently persuasive.