The Bush administration yesterday endorsed legislation for a referendum on the future of Puerto Rico, but several island officials said later that administration reservations about the wording of a Senate bill on the referendum will lessen prospects for the measure's passage.

"I'm more convinced than ever that there is going to be no plebiscite," said Resident Commissioner Jaime B. Fuster (D-Puerto Rico), the island's non-voting delegate in the House.

Fuster said objections Attorney General Dick Thornburgh raised yesterday with the Senate Energy and Natural Resources Committee could force the committee to rewrite "the entire bill."

For two years, island officials have been seeking congressional support for a referendum that would allow voters on the Caribbean island to decide whether Puerto Rico should become the 51st state, seek independence or remain a U.S. commonwealth. The island, which has a population of 3.6 million, has been under the U.S. flag since 1898, when it was ceded by Spain after the Spanish-American War.

Officials from the administration of Gov. Rafael Hernandez Colon were downcast after yesterday's hearing, saying the questions from Sens. Bill Bradley (D-N.J.) and Don Nickles (R-Okla.) illustrated that members of both parties harbor strong reservations about the issue. Bradley charged the bill "slipped" a controversial issue -- whether English would be mandated for the largely Spanish-speaking island. Nickles questioned whether Congress should let the issue of statehood be on the ballot.

Committee Chairman J. Bennett Johnston (D-La.) and administration officials remained upbeat. Repeating his desire for Congress to resolve the referendum request this year, Johnston said he will attempt to mark up the bill later this month.

Charles G. Untermeyer, White House personnel director and the leader of the administration forces on the issue, said he has begun meeting with Republican senators on the issue, the first sign of direct White House lobbying on behalf of Puerto Rican statehood. President Bush has endorsed statehood and called for a referendum two years ago.

But Untermeyer said many GOP legislators are raising political objections, fearing that, as a state, Puerto Rico would elect an overwhelmingly Democratic congressional delegation.

Thornburgh yesterday urged the panel to make statehood effective shortly after the vote, an action that would require additional legislation. The Senate measure proposes a five-year period to phase in statehood privileges and to end the island's favored status under federal income tax laws.