It's taken nearly nine years, but large-scale commercial redevelopment of the World Trade Center site is tantalizingly close to taking off. But everything depends on the outcome of two ongoing, parallel negotiations involving the Port Authority -- neither of which is a sure thing.

The Port Authority's 1 WTC is pushing skyward, and Larry Silverstein's 4 WTC should start major construction soon. The towers together would restore about half of the Twin Towers' 8.8 million square feet of office space.

It's less than the city needs but more than political and bureaucratic inertia once seemed likely to bring forth -- and, if successful, would keep hope alive that two more great towers planned for the site will go up eventually.

PA Executive Director Chris Ward is backing agreements with developers to ensure that 4 WTC rises and to make 1 WTC viable. But either deal could be undone by the PA's New Jersey side or by rogue commissioners from either state.

The more critical issue involves 4 WTC. Without a deal for the PA to provide full "backstop" financing (i.e., loan guarantees), Silverstein can't build the $1.8 billion, 61-story, state-of-the-art structure above its current, six-story steel base.

The PA and Silverstein announced a tentative agreement in March, with a binding contract to be signed within 120 days -- but indications now are that a final deal won't happen by the PA's next board meeting, on Aug. 5.

Ominously, New Jersey Gov. Chris Christie has sounded less than fully committed to the plan -- and one PA commissioner, Henry R. Silverman (a New York appointee and until recently the agency's vice chairman), has grumbled that the PA can't afford it.

Even a delay could prove devastating. Silverstein hopes to finish 4 WTC by 2013, when New York's aged office stock will seem even more antiquated to companies requiring state-of-the art facilities. The project's 1.8 million square feet will help meet the inevitable demand.

But Silverstein must convert $1 billion in Liberty Bonds (now in escrow) into a construction loan by year's end to meet the 2013 target -- or to build at all. (The tax-exempt bonds would let him borrow more cheaply; the PA backstop would protect borrowers against default.)

Yet he can't sell the bonds until the PA agrees to the financing terms. And given the history between the sides, nothing should be mistaken for done until it's done.

At 1 WTC, the PA and Douglas Durst are trying to hammer out a partnership arrangement, announced last month, to give him a $100 million stake in the $2.6 billion project. That would "brand" the long-troubled tower in the name of one of the city's most successful private developers and make him responsible for leasing and managing it.

Durst would have a far stronger chance of making 1 WTC commercially viable; the PA has neither heart nor skill for the task. But, the New York Observer reported, the PA wants the right to boot Durst simply if it doesn't like how things are going. Such a prenup hardly suggests confidence and could end up putting the PA at war with Durst, as it's long been with Silverstein.

Private-developer expertise is needed because nearly all of 1 WTC's 2.6 million square feet are up for grabs. The oft-published assertion that a federal agency and a state agency are "committed" to 1 million square feet is false: Neither lease has been signed three years since they were first announced.

Yet private-sector interest in 1 WTC is strong. As I first reported, Bank of New York Mellon is considering moving its headquarters there. So is Conde Nast, a tenant at Durst's 4 Times Square.

Durst could also help break the logjam with the larger of the stalled government-agency leases. The Federal General Services Administration, which would take 600,000 square feet, happens to be a tenant at his 1133 Sixth Ave.

The PA must overcome internal bickering and complete the deals with Silverstein and Durst. Otherwise, the "World Trade Center" will consist of a sprawling memorial and one lonely, near-empty office building amid empty lots.