ALBANY, April 14 - The State Assembly passed a plan Wednesday to soften New York's mandatory sentences for drug crimes, acting rapidly on the measure after returning from a 12-day vacation.

The proposal faces opposition in the Republican-led Senate, however, and Assembly leaders called Wednesday for a conference committee to work out the two chambers' differences on revising what have come to be known as the Rockefeller drug laws.

At the heart of the Assembly's bill, which the chamber also passed in June 2003, is a plan to give judges the discretion to send people facing drug charges into treatment programs instead of prison, said Jeffrion L. Aubry, a Democratic Assemblyman from Queens, who devoted much of his career to changing the laws.

The measure from the Democratic-led Assembly would also double the minimum weight of narcotics that would bring criminal charges (for example, to four ounces from two ounces of cocaine or heroin for an A-1 felony charge for sale); and it would lessen the mandatory minimum prison sentences while increasing penalties for violent or predatory drug kingpins, Mr. Aubry said.

Still, any chance of reaching agreement with the Senate and passing legislation to send to Gov. George E. Pataki seemed to fade quickly Wednesday. John E. McArdle, a spokesman for Joseph L. Bruno, the Senate majority leader, said that although convening a conference committee had not been ruled out, many in the Senate were frustrated by a belief that the Assembly had walked away from past agreements.

"It is not a bill we support; it does not address many of the issues that need to be addressed," Mr. McArdle said of the bill that the Assembly passed Wednesday 87 to 56. "We pass legislation year after year after year on Rockefeller drug law reform."

Perhaps more than any other issue, the topic of how to change the laws Gov. Nelson A. Rockefeller enacted in 1973 underlines Albany's legislative paralysis. While Governor Pataki, Mr. Bruno and Sheldon Silver, the Assembly speaker, all agree that change is needed, they cannot agree on how to ease sentences, how to treat those already sentenced and whether to take decisions about a defendant getting treatment or a prison term out of the hands of prosecutors.