Puerto Rico holds its first primary election of delegates to the national Democratic Party's mid-term convention today, but the voting has become little more than an exercise in futility.

The primary was first billed as a preview of a status plebiscite, with pro-common wealth and pro-statehood factions vying for control of the local branch of the Democratic Party. But events have drastically altered the character of the election.

The pro-commonwealth faction, the Popular Democratic Party, declared in June that it would boycott the primary.

One reason for te PDP withdrawal was a split in its ranks between those who favored voting and othets who argued that the primary would entangle the party in U.S. national politics. A basic tenet of the PDP and other supporters of commonwealth status is Puerto Rico's political autonomy.

The PDP withdrawal left statehooders with control of both the Democratic and Republican parties' local branches.

But the character of the primary changed a second time Oct. 5, when the commonwealth Supreme Court declared public funding of the primary unconstitutional.

The decision, in effect, reduced this year's primary to a small, private affiar and may for practical purposed do the same to what originally was slated as a crucial presidential primary in 1980.

The legislature had allocation $600,000 for expenses in this year's primary. The Puerto Rico Socialist Party filled suit, charging that the Puerto Rican constitution prohibits use of public funds for anything but "public purposes."

In a 6-to-2 vote, the commonwealth Supreme Court, which is composed mostly of commonwealth advocates, ruled that the primary is not a legitimate public purpose because it would push Puerto Rico toward a closer political relationship with the United States.

The statehood faction of the party has collected about $30,000 in private donations to go ahead with today's voting, to select 25 delegates to the Democratic Party's mid-term convention in December.

The major question in the contest has been reduced to only how many pro-statehood voters will turn out. Statehood leaders are predicting a showing of about $200,000 and say that less than 100,000 will constitute a failure.

The selection of Democratic delegates first became a status contest when the Democratic National Committee increased Puerto Rico's delegates from seven to 22, making it the 25th ranking delegation in the United States.