JIMMY CARTER is fast on the way to becoming the patron of Puerto Rico as the 51st state. He is spurring a process that, as early as 1981, may bring the 3 million people of the island commonwealth to a new vote on their "status," with choices including independence and a modified version of commonwealth as well as statehood. Just the other day he pledged that whatever status Puerto Ricans choose, "it will be yours." In fact, it will be up to the Congress, which has not previously had to deal with the question of taking into the union a racially mixed, Spanish-speaking territory at a very different level of development from the "mainland." But in Puerto Rico the stathood tide is unquestionably flowing.

The roots of Mr. Carter's interest in providing yet another occasion for Puerto Ricans to select their status - in 1952 and 1967 they supported commonwealth - are unclear. His aides suggest it is a matter of devotion to the principle of self-determination and of respect for Spanish-speaking people. But they acknowledge that the trigger was a political debt accumulated to Franklin Delano Lopez, who working through the San Juan ADA chapter, swung some early Puerto Rican delegate votes to Mr. Carter in 1976 while most delegates were going with Sen. Henry Jackson, chairman of the congressional committee (Interior) that oversees commonwealth affairs.

Mr. Lopez, it turned out, was an ardent statehooder. And he happened to make his move at a moment when PuertoRico's commonwealth status was becoming a scapegoat for various political and economic grievances:

Mr. Carter's political blessing thus was transformed on the island into a presidential blessing for statehood. Whether Mr. Carter and his White House staff have been entirely aware of that process is in dispute. What is not in dispute is that the process has spread consternation among those Puerto Ricans (formerly in the majority) who would rather improve commonwealth, with its considerable economic protections, than give it up for the full political rights offered under statehood. (Under commonwealth, Puerto Ricans don't pay federal income taxes and don't vote.)

The Democratic National Committee has just made it possible for the Lopez slate to run unopposed in a party primary next fall to select Puerto Rico's delegation to the 1980 party convention. The long-dominant, pro-commonwealth Popular Democrats, traditionally the Puerto Rican political organization closest to mainland Democrats, is in disarray. The Puerto Rican party closest to the GOP is also in the statehooders' ranks.

Well before mainland Americans get to weigh the expected Puerto Rican petition for statehood, however, another group will consider the merits of the Puerto Rican complaint. IT's the U.N. Decolonization Committee, which is annually asked by Cuba, long Puerto Rico's self-appointed big brother, to stigmatize Puerto Rico as the United States' Colony. Until last year, only the few Puerto Ricans favoring independence seconded the Cuban motion. But last year mainstream Puerto Ricans joined the chorus, mainly, it seems, to display their discontent with the workings of commonwealth; nonetheless, the Cuban petition did not carry. This year, they will be back, and changes in the decolonization committee make it possible that for the first time the United States may be condemned as a colonial power. It was partly to head off that embarrassment that Mr. Carter hardened his support for Puerto Rican self-determination the other day.

In brief, in Puerto Rico the statehood drive is on. Though the question is virtually unexamined in American political life, it has the Carter administration's strong political and ideological support. Ready or not, mainland Americans will have to start considering in the next few years how they would feel about Puerto Rico as the 51st state.