IT HAS BECOME AN ANNUAL United Nations ritual for Cuba to declaim that the United States holds Puerto Rico as a "colony" and for the United States and Puerto Rico to respond indignantly that Puerto Rico enjoys freedom and the right of self-determination, and it's none of the U.N.'s business anyway. So why did representatives of all the island's parties - including the party favoring the current status of commonwealth, the statehooders and the small independence party - turn up at the U.N. the other day essentially to support the Cuban attack?

What is on its face a diplomatic embarrassment for the United States is in fact a demonstration of the vigor of demoratic politics in Puerto Rico. For once again Puerto Ricans are hotly debating what relationship to the "mainland" they wish to have. The last time out, in a referendum in 1967, they reaffirmed the commonwealth tie. But nagging difficulties in that relationship, accentuated by economic woes, revived the issue in the last presidential term. Mr. Ford gave the commonwealth two body blows, first by ignoring the detailed report of the commission he set up to explore ways to improve it, and then by suddenly and inexplicably coming out for statehood just as he left the White House, Mr. Carter kept up the momentum by telling Puerto Ricans he would support self-determination "whatever your choice may be."

The Puerto Ricans showed up in New York, then, to make a record of their dissatisfaction with the status quo. That will be useful to them, they variously calculate, as the status debate hums along at home. Even the pro-commonwealth party now takes the position that the existing unimproved commonwealth tie is inadequate.

It will irritate some Americans to see a U.N. committee discussing a matter that all Americans, including, of course Puerto Ricans, believe is an internal affair. (Interestingly, some administration officials wonder privately if the United States would not gain, in showing it has nothing to hide, by accepting the U.N.'s competence to discuss the question.) But even if the Cuban resolution passes - it gratuitously commends self-determination and demands release of five jailed Puerto Rican nationalists - no one expects it to have any real effect on the status delibertations in Puerto Rico.

So far there has been little mainland attention given to the growing possibility that in the next few years Puerto Ricans may decide to ask Congress to become the 51st state. We will reserve discussion on the merits. But the question is coming.