For 12 years, a threat hung over Plum Island like the Sword of Damocles: that one day the federal government would sell the unspoiled preserve into private hands and leave it to an unknown fate.

On Monday, that threat disappeared when Congress repealed two laws that mandated the sale, all but guaranteeing that the island will not be developed.

"This is a huge hurdle," said Chris Cryder, land campaign manager for Save the Sound, a group that has taken the lead in working to keep the island publicly owned.

The 840-acre island, which lies eight miles south of the region's shoreline, is home to the Plum Island Animal Disease Center but has been off-limits to the public for decades.

The lab is scheduled to move to Kansas in 2023, after which the island will likely be declared surplus federal property. Normally that triggers a disposition process that offers the land first to other federal agencies, then to the state where it's located, then to local authorities. If none of them are interested, it's put on the open market.

But laws passed by Congress in 2009 and 2012 inverted the process, requiring that the private sale come first to help fund the lab's replacement in Kansas. A coalition of 110 groups with a broad range of interests has been waging a bipartisan fight to keep the island out of private hands.

Their efforts resulted in language repealing the sale laws being included in the enormous year-end spending bill approved on Monday. The 5,593-page bill, which according to the Associated Press is by far the longest in the history of Congress, includes the $900 billion coronavirus relief package and also funds the federal government. It passed both houses by veto-proof majorities and awaits President Donald Trump's signature.

"Today we can finally and fully celebrate preserving Plum Island," said Sen. Richard Blumenthal, D-Conn. "This exquisite environmental treasure has been spared a headlong rush to sell to the highest bidder."

U.S. Rep. Joe Courtney, D-2nd District, noted that the House had passed legislation to save Plum Island for five years in a row. It had never passed the Senate.

"The final negotiated omnibus spending package released (Monday) will get the job done once and for all," he said.

With sale and development now off the table, the island will go through the normal disposition process.

"It opens a window for a number of different entities to conserve Plum Island," Cryder said.

In addition to the repeal, the bill provides $18.9 million to clean up the island, which Cryder said will be done by the federal government under the oversight of the New York Department of Environmental Conservation.

The repeal comes six months after the Preserve Plum Island Coalition released a 72-page report that offered its vision for a future that would keep the island's existing features intact. These include laboratory facilities and a nature preserve, and historic preservation of a lighthouse and of Fort Terry, an early 20th-century fortification. Controlled public access was also part of the plan.

The report noted that zoning had decreased the island's anticipated value on the open market. It envisioned the State of New York as owning the island, which is part of the town of Southold.

Cryder said he still sees New York as the entity best suited to be the owner. But the return to the disposition process would give other federal agencies the first shot at taking the island over.

When the report was written, federal interest was considered unlikely because the Trump administration was focused on getting rid of federal land, according to Greg Jacob, a policy adviser for the Nature Conservancy in New York, another group in the coalition.

But federal agencies could have a different philosophy under the Biden administration and possibly be interested in acquiring the island, Jacob said. Though he also believes New York would be the island's best steward for conservation, "there's multiple roads to get to where we need to go."

The coalition had been working for several years to insert language repealing the sale laws into broader measures like appropriations bills to improve the chance of passage.

The legislative path that finally succeeded began earlier this year with a bill authorizing the Plum Island lab's replacement, the National Bio and Agro-defense Facility in Manhattan, Kan., Jacob said.

That bill was introduced by Sen. Pat Roberts, R-Kan., then blocked by Blumenthal, who asked Roberts to include the repeal language, Jacob said.

Roberts, who is retiring from Congress, declined but made clear he wanted authorization of the Kansas lab to be part of his legacy, Jacob said.

That kept negotiations over the repeal language alive, and in the end-of-year rush to pass an overall spending bill, a deal was struck in the last couple of weeks that allowed the sale laws to be repealed.

"It boiled down to we were able to give him something he wanted, and he was able to give us something we wanted," Jacob said.

Jacob bought a bottle of beach plum gin and said he drank a toast while watching the votes in Congress. Plum Island was named for the beach plums that grew there.

"It was a good pre-Christmas celebration," he said.

j.ruddy@theday.com