The fate of the Javits Center expansion is in the hands of the Democratic speaker of the Assembly, Sheldon Silver, who is poised to extract concessions from Governor Pataki and Mayor Bloomberg in the lead up to a critical vote on the $1.7-billion project.

Once again Mr. Silver, as one of three voting members of an obscure state board that has the ultimate say over major state development projects, is in position to leverage his power to his advantage.

The Public Authorities Control Board, comprised of Mr. Silver and representatives of Mr. Pataki and Republican Senate majority leader, Joseph Bruno, is expected next week to rule on plans to add more than a million square feet to the convention center on the West Side.

Mr. Bruno and Mr. Pataki are in favor of the expansion, but Mr. Silver is in a position to torpedo the plans by casting a vote against it. He could also remove it from the board’s agenda for its July 19 meeting, a move that would further delay construction of an important legacy project for the Pataki administration.

In Albany, Mr. Silver is famous for waiting until the last minute to show his hand during budget negotiations. The strategy has served him well, particularly during last year’s battle over the proposed Jets Stadium on the far West Side.

Less than two weeks before Mr. Silver cast his vote to derail the Jets Stadium, Messrs. Pataki and Bloomberg announced plans to spend more than $800 million on a slew of projects for Lower Manhattan, which Mr. Silver represents in the Assembly, including money for a new school on Beekman Street, fixing up waterfront parks, and adding affordable housing.

State officials say that Mr. Pataki is set next week to put the Javits Center expansion projects on the agenda for final approval in front of the control board. Mr. Silver’s office said yesterday that the speaker is reviewing the plans, which have been virtually unchanged since January.

There are signs that Mr. Silver’s support is by no means guaranteed. Two sources close to the Javits Center project said they had heard Mr. Silver was upset that Mr. Bloomberg had not consulted him before the city offered the Metropolitan Transportation Authority $500 million for development rights for the MTA’s 26-acre Hudson rail yards on the far West Side, the site of the failed proposal to build a Jets football stadium. Mr. Silver may be withholding his blessing on the deal to express his anger over the city’s surprise offer, the sources said.

A spokesman for Mr. Silver, Charles Carrier, said the speaker is seeking more details on the city’s bid on the rail yards.

Some observers say that Mr. Silver is merely exhibiting his trademark negotiating style.

The director of New York Civic, Henry Stern, an outspoken critic of Mr. Silver, said the speaker “negotiates until the last minute, and tries to get everything he can for himself or his district.”

Mr. Stern said: “The motto is there are no free rides. Even if he likes it, he will make you pay for it. The merits are incidental.”

A Republican assemblyman from Canandaigua who represents the Assembly minority leadership as a non-voting member on the control board, Brian Kolb, said, “The speaker has always been a very good poker player, and that has served him well.”

Asked if Mr. Silver’s strategy has effectively served the state and the city, Mr. Kolb said, “In my view, no.”

The expansion of the Javits center would make New York’s convention center the fifth biggest in the country, up from the 19th slot. Aside from Mr. Bloomberg, the plan’s backers include the City Council speaker, Christine Quinn; unions involved in the construction trade, and the city’s powerful tourism industry.

Critics of the current plan say that it is too small and too expensive. They cite an odd vertical configuration they say is unsuitable for convention space, and they question the state’s decision to sell the development rights of the block between 33rd and 34th streets and Eleventh and Twelfth avenues, moving the truck marshalling yards to the north.

The former chairman of the Convention Center Operating Corporation, Robert Boyle, was removed by Governor Pataki in December for opposing the latest changes.

The president of the Partnership for New York, Kathryn Wylde, said she thought it was likely that Mr. Silver was considering the plan on its merits. “Everything I know is that this is nothing to do with negotiating politics and everything to do with real concerns about whether this is the best possible public investment in getting a first-class facility,” Ms. Wylde said.

Timing is key for the Javits project. One official who is familiar with the Javits plan said proponents of the convention center expansion are worried that pushing back the expansion further could delay it until the next administration takes over in January. The next governor may not share Mr. Pataki’s vision for the site.

The state attorney general, Eliot Spitzer, a Democratic front-runner in the governor’s race, has not expressed his position on the plans. A spokeswoman for Mr. Spitzer, Christine Anderson, said the attorney general would comment on it today.

Lloyd Constantine, an attorney who is a donor to Mr. Spitzer’s campaign and a close friend of the attorney general, was one of two appointees to the state’s Convention Center Operating Corporation who voted last month against the latest Javits plan. Mr. Constantine said last month that the designs for the center would become obsolete even before construction began. Mr. Silver’s appointee to the corporation, Ronald Goldstock, also voted against it.Thirteen others on the corporation board voted in favor of the plan.

The Municipal Art Society, a planning group that opposes the project design and configuration, is suing the state to halt the project. The group claims the state failed to update its environmental impact statement. The state has yet to respond in court to the suit.

In December 2004, the state Legislature approved the project, which will cost the city and state $350 million each. In September 2005,the state authorized $800 million in bonds, to be backed by a $1.50 a room hotel tax collected by the Hotel Association of New York City. In January,the state announced several revisions to the plan,and said the project’s costs would rise from $1.4 billion to $1.7 billion. The state hopes the project will be completed by 2010.