The Giants' long-term lease in New Jersey, which runs to 2026, includes language requiring New Jersey to maintain Giants Stadium as a "state-of-the-art" NFL facility — and that could be a way for the Giants to get out of it, the source said.

New Jersey has failed in that duty to maintain the stadium, giving the Giants a legal opening to bail out of the deal, the source involved in the talks said.

The Jets could want the Giants as partners for a West Side stadium to reduce the Jets' cost, possibly boost the value of their bid for the MTA property, and create a public-relations coup in bringing both teams to the Big Apple.

But the Giants team made it clear yesterday they'd still prefer to build a new stadium of their own near their existing arena in the Meadowlands.

"Our only goal in this entire process has been to negotiate a deal with the state of New Jersey," said John Mara, the Giants' executive vice president and chief operating officer.

Mayor Bloomberg — who has taken relentless heat from opponents of the West Side stadium, which he hopes will also be used for the 2012 Olympics — also wasn't optimistic about winning the Giants.

"The Giants can't move to New York. They have a contract with New Jersey," Bloomberg told The Post.

Weeks ago, negotiations between the Giants and operators of the Meadowlands complex seemed to be going well, with both sides heading toward a deal for a spanking new $750 million, 80,000-seat stadium.

But the talks foundered in the last few days as New Jersey acting Gov. Richard Codey, anxious to balance the state budget, proposed imposing a 6 percent sales tax on luxury-box rentals at the stadium.

When the Giants balked at that proposal, Codey agreed to omit it from this year's state budget — but he wouldn't agree to exclude it from future years' budget plans.

The Giants also are wary of Xanadu, a $1.3 billion entertainment and shopping complex to be built near the stadium. The Giants are worried that Xanadu visitors might make parking more difficult for their fans on game days.

"Unfortunately, we do not have a deal, and it doesn't look very likely that that is going to change," Mara said last night.

Meanwhile, the West Side proposal picked up another labor endorsement yesterday when the union representing police detectives backed the plan.

"If our great city had succumbed to the opposition of Yankee Stadium, think of all that would have been lost in revenues as well as lifelong memories," said Michael Paladino, president of the 15,000-member Detectives Endowment Association.

"The project is a long-term investment . . . and if we have any vision at all, let it not be 'tunnel vision,' " Paladino said.