The state is expected today to open the West Side rail yards to bidding by developers, a crucial early step in the process of transforming the 26-acre Hudson Yards site, according to multiple people familiar with the plans.

The announcement of the request for proposals, which Governor Spitzer is expected to deliver, comes just less than two years after plans for a new Jets and Olympic stadium on the site were defeated.

The state is expected to invite developers to submit plans for up to about 12 million square feet of new commercial and residential development and a generous amount of open space — by contrast, Brooklyn’s $4 billion Atlantic Yards project is planned to have 8 million square feet — allowing for a giant new complex to sprout in a corner of the rapidly transforming Hudson Yards district.

“This is a lot of development potential,” the president of the Real Estate Board of New York, Steven Spinola, said. “There’s development teams that are clearly ready to take the RFP and respond.”

For months, the city and state have been working to sort out the details of the request for proposals, which sets limits and guidelines for developers submitting bids. Among the issues under debate were provisions for “affordable” housing and how to handle the High Line, an elevated former rail line that is being transformed into a park in neighborhoods south of the yards.

The MTA, which owns the yards, has been seeking to keep the requirements on developers to a minimum so as to raise the value of the site, people familiar with the process say.

Elected officials representing the area have been pushing for permanent affordable housing on the site, a provision that was not included when the initial guidelines were announced in early May.

Five of the city’s largest development companies, some of which are said to be working in teams, are creating plans for the site.