When a government breaks a law over and over again, it does not render the law of no effect. Those who believe in the rule of law cannot choose what to obey and what not to obey. Under our Constitution, a treaty is the supreme law of the land. One such treaty that we signed and ratified is the Charter of the Organization of American States. It provides with utmost clarity:

''No State or group of States has the right to intervene, directly or indirectly, for any reason whatever, in the internal or external affairs of any other State.''

In his eloquent support of the Arias peace plan, Charles Robb {op-ed, Jan. 24}, a distinguished Virginia lawyer and former governor, seems to pass over our treaty obligation not to intervene in the ''internal or external affairs'' of our Latin American neighbors. Gov. Robb is in the company of a multitude of lawyers and statesmen, past and present, in casting aside the law of the land and advocating aid to the Nicaraguan contras.

The fact that we have broken the law many times before in the countries of Latin America does not mean that we are no longer bound by the law. Article VI of the U.S. Constitution says we are. Perhaps the time has come to rid ourselves of our scofflaw image. If we don't intend to obey the law, we should seek to have the treaty abrogated or amended. This is a time when many on Capitol Hill and in the administration find themselves freely accusing the Soviet Union of disregarding treaties. If we are guilty of the same thing, should we not try to cleanse our own reputation?

When I put the matter of breaking the OAS treaty to a prominent officer of the Reagan administration recently, he responded that there were ''juridical opinions'' to the contrary. Evidently, he had in mind security arguments connected with the Rio Treaty. One would think that these were covered by the clause ''for any reason whatever'' in the OAS charter. If they are not covered, then we should clarify matters by invoking the Rio Treaty. The same official noted that it might have been a mistake not to have done so.

Aside from other issues in the current debate, the status of the United States as a law-abiding nation is worth defending. MURAT W. WILLIAMS Washington The writer was U.S. ambassador to El Salvador in 1961-64.