Legislation that would allow the residents of Puerto Rico to decide their island's political fate appeared dead yesterday after a key senator announced he would invoke his power as a committee chairman to block Senate consideration.

Moments after the House endorsed a referendum bill by unanimous voice vote, Sen. J. Bennett Johnston (D-La.), the chief sponsor of similar legislation in the Senate, announced there would not be time to consider the bill during the current session of Congress. "It would be impossible to take it up, even if we accepted it in its present form," Johnston told the Senate.

The legislation, supported by President Bush and all three island political parties, would authorize a voter referendum next September on whether Puerto Rico should become a state or an independent republic, or remain a U.S. territory with improved federal benefits.

The issue of the island's political status has dominated politics in Puerto Rico since it became a U.S. possession in 1898 as one of the spoils of the Spanish-American War. The Philippines and Cuba, which the United States also acquired at the time, later were granted independence, but Puerto Rico's status has remained unsettled.

Supporters of statehood for the District of Columbia have sought unsuccessfully to link their proposal to the Puerto Rican voter referendum.

Johnston's decision to reject the House bill was not a complete surprise. Aides said he told Puerto Rican leaders two years ago he would insist that any referendum bill fully define what the three political conditions would mean to the 3.7 million residents of the island, many of whom are dependent on federal welfare programs.

But the House decided to sidestep that issue, saying Congress should define the island's political status only after voters there indicate which of the options they favor.

Because Johnston was involved in a closely contested primary until last week, the referendum sponsors did not confer with him until this week. Noting that Johnston's own bill was still in committee, they expressed hope he would support the House bill as the only way to hold the promised referendum next year.

But aides said Johnston was unconvinced that, after three decades of congressional debate, the measure had to move so rapidly in the final days of the session. "The senator's position is that we have waited 30 years. Another year is not going to matter," said one Johnston aide.

Yesterday afternoon House members hailed their unanimous vote as ending an era of colonialism in U.S. foreign policy.

In nearly an hour of speeches praising their proposal, only Rep. Bill Richardson (D-N.M.) raised the possibility that the bill might be endangered.

"The biggest enemy of this bill is the calendar," Richardson said, appealing directly to Johnston to let the Senate act promptly on the measure. "There is a dire possibility . . . that this bill might die."

House backers acknowledged that their strategy was risky. There was no organized opposition to the referendum in the House, but a number of senators have reservations.

Many senators from states receiving large amounts of federal aid are skeptical, perhaps because Puerto Rico would be entitled as a state to a larger share of federal welfare programs.

Others said that despite the president's support for the referendum, Republicans fear the island would send two more Democrats to the Senate.

Resident Commissioner Jaime B. Fuster (D), the island's non-voting delegate to Congress, expressed concern that Johnston's action may have delayed any Puerto Rico referendum well beyond 1991.

All the island's political offices -- from governor to the municipal level -- will be up for election in 1992, and Fuster said none of the political parties want the referendum to be held in an election year.

Staff writer Don Phillips contributed to this report.