ALBANY — In a setback for Mayor Bloomberg, state lawmakers are thwarting the city’s plan to construct a waste transfer station at the Gansevoort Peninsula.

The Democrat-controlled Assembly is leaning against backing legislation that would allow the city to build the facility on a small section of the Hudson River Park near West 14th Street.

Permission to build the Gansevoort station was the only thing Mr. Bloomberg needed from Albany to move ahead with his long-term plan to overhaul how the city handles its trash.

Opposition in Assembly could force the administration to tear open the plan, which was approved by the City Council after years of contentious debate, and make numerous adjustments.

The Bloomberg administration sought to gain approval of the sanitation facility by appealing directly to members of the Assembly’s black and Puerto Rican caucus, arguing that the city’s plan would improve air quality in their districts, many of which are outside of Manhattan, by cutting down the number of trucks hauling trash to other boroughs from the island.

The thinking by the administration was that if they could get the caucus behind their plan, they could overcome opposition by three West Side Manhattan lawmakers whose districts overlap with the park. Lawmakers, in turn, questioned whether the intent was to sow divisions by pitting white members opposed to the legislation against their minority colleagues.

Regardless of the goals, the mayor’s strategy does not appear to be working. The caucus, which includes more than 30 members from the city, said yesterday it is declining to take a position on the legislation, effectively siding with the Manhattan lawmakers, Richard Gottfried, Deborah Glick, and Linda Rosenthal.

“They just don’t think it’s the right place,” the chairman of the Assembly’s Ways and Means Committee, Herman “Denny” Farrell, said. As Democratic county chairman of Manhattan, Mr. Farrell is one of the caucus’s most influential voices.

Echoing their reservations, the Democratic speaker of the Assembly, Sheldon Silver, yesterday urged Mr. Bloomberg to consider alternative locations for the trash station.

The caucus’s decision not to support the Gansevoort facility leaves the Bloomberg administration without a powerful ally to carry the bill, sponsored by Adriano Espaillat of Upper Manhattan.

Lawmakers said Mr. Bloomberg might have overestimated his influence over black and Puerto Rican lawmakers and the willingness of lawmakers to break ranks with their conference on an issue involving local land use.

“We try to respect one another’s district,” Mr. Gottfried said. “In park bills, that has been especially emphatic.” He said he warned other members: “If the mayor gets to do this to us, next time, he’ll do it to you.”

He and others opposed to the current location of the stations have suggested other alternative Manhattan west side locations, including at Pier 76 at 36th Street. The Bloomberg administration contends the site could not easily accommodate a transfer station, unlike the Gansevoort Peninsula.

The Gansevoort waste transfer station “is critical to ensuring that every borough, including Manhattan, is responsible for processing its own waste and recyclables rather than exporting them by truck to another borough,” Mr. Bloomberg wrote in a letter to lawmakers, urging them to pass the amendment. The mayor said that the proposed facility, occupying two-thirds of an acre, would not “meaningfully reduce the park’s 550 acres” and would sit “almost entirely over water.”