The Popular Democratic Party, advocate of Puerto Rico's current commonwealth relationship with the United States, has been thrown into confusion by the surprise resignation of its leader Rafael Hernandez Colon.

Leaders of political parties advocating statehood or independence for the island interpreted his resignation as another sign of the pro-commonwealth party's decline since it lost the 1979 elections to parties pressing for statehood.

Hernandez Colon's announcement was broadcast late Wednesday, just six days before the party's general assembly July 25, which was expected to rubberstamp his reelection as party president. No other potential leader has emerged to take his place.

"Hernandez Colon acted like a captain abandoning a sinking ship," commented House Majority Leader Jose Granados Navedo, of the pro-statehood New Progressive Party.

Independence Party leader Ruben Berrios predicted the Popular Democrats' demise and appealed to commonwealth advocates to join with independence forces.

Hernandez Colon did not reveal his reasons for resigning in his 15-minute radio speech, saying only that he needs to communicate "the vision that through the years I have held of what we Puerto Ricans want to be."

In 1976, the 41-year-old former governor, who led the party 10 years, was blamed for its election defeat.

He immediately launched an 18-month party re-organization, took on his challengers publicly and asserted his control as president of the party.

Statehood and independent leaders saw Hernandez Colon's sudden withdrawal as the result of a "coup d'etat" led by the party's 80-year-old founder Luis Munoz Marin.

Just two days before, Munoz had announced his return to public life, after 14 years in retirement, to actively campaign against statehood.

Munoz said he had learned that Hernandez Colon might withdraw and had decided to make his comeback "to avoid an internal struggle over candidacies."

The former four-term governor rejected the possibility of being a candidate for the PDP presidency or the governorship in 1980.

Opposition leaders said they expect Munoz to signal who will be the next PDP president.

The Popular Democratic Party was almost the sole governing political force in Puerto Rico for more than 40 years until 1976. The party lost the governership only once before, in 1968, because of an internal split, while it retained control over the legislature.

In 1976, for the first time, the pro-statehood party won an unexpected victory - claiming both the governorship and the legislature.