The leaders of Puerto Rico's largest political party have announced that they will support a proposed referendum on the island's political future, but only if House and Senate leaders can agree on the enabling legislation by Feb. 19.

Without such an agreement, Popular Democratic Party chiefs decided at a weekend meeting, they will withdraw their support for congressional efforts to resolve the island's status until 1993, said Resident Commissioner Jaime B. Fuster, the island's non-voting member of Congress.

The deadline threat seems a calculated risk certain to cloud the announced plans of Sen. J. Bennett Johnston (D-La.) to press for legislation authorizing a referendum sometime in 1991.

In the final days of the 101st Congress, the House approved legislation calling for such a referendum, but Johnston, who chairs the Senate committee with responsibility for territorial issues, deferred any Senate action until next year.

The planned legislation would allow voters on the island to decide in 1991 whether they want Puerto Rico to become a state or an independent republic or remain a U.S. commonwealth.

Johnston's timetable has been greeted with skepticism by some Puerto Rican politicians, who point out it is unlikely that both houses can agree on such a complicated issue in time for a referendum next year. Even a delay until 1992 might be inadequate, because virtually all the island's political offices will be up for election then and Democratic party leaders have said there may not be enough time to hold both local elections and a separate referendum in the same year.

Fuster noted that while leaders in both the House and Senate are committed to the concept of a referendum, they remain far apart on how the vote should be conducted. The Senate has pressed for a referendum that would implement the voters' choice automatically, but the House has endorsed a two-step process that would require new legislation to implement the option voters pick.

Fuster acknowledged that his party's vote is an effort to force congressional leaders to make an early decision on the issue.

"This is the only way we can break the stalemate," he said. Without an early agreement, Fuster said, the proposed referendum -- which has been endorsed by President Bush -- will probably stall again next year between the House and Senate.

Jose Martinez, executive director of the Commonwealth of Puerto Rico's Washington office, described the party resolution as "a last-chance opportunity" that the Popular Democrats would give their governor, Rafael Hernandez Colon, to press for the referendum, which he first proposed in 1989.

Johnston's office had no immediate comment.