Congressional Republicans’ immediate rejection of an Obama administration plan to close the Guantánamo Bay military prison means yet another part of the president’s legacy could be decided by the courts.  

Unable to convince a GOP-run Congress to support his plan and short on time to make good on his pledge to shutter the terrorist detention facility, Obama’s last option is to go it alone. And given recent history, that’s why experts expect the lawsuits to begin.  

The White House used executive actions on high-profile issues such as immigration and reducing power plant carbon emissions. Courts put those efforts on hold. And experts say any similar plan to shut Guantánamo could ultimately reach the U.S. Supreme Court, which currently only has eight of its nine jurists.  

Obama administration officials have not acknowledged that an executive action to close the facility or transfer a large majority of its remaining population is under consideration. But, notably, they also have not explicitly ruled it out.  

Even before Obama outlined the plan on Tuesday, Republican leaders dubbed it “illegal” and served notice it has almost no chance of being enacted. But Obama ended his remarks with a defiant tone .  

“I don’t want to pass this on to the next president,” Obama said, flanked by Vice President Joseph R. Biden Jr. and Defense Secretary Ashton B. Carter. “I am absolutely committed to closing the detention facility at Guantánamo.”  

One lawyer who represents a Gitmo detainee, who requested anonymity so he could speak candidly, said he “strongly believes Obama will use his executive authority to close the prison.”  

“He sees it as a stain on the U.S. judicial system. And, remember, he’s a constitutional scholar,” the lawyer said. “He will use every aspect of his executive authority to close it — and soon.”  

Decades of legal and national security scholarship shows the federal courts — but especially the Supreme Court — typically sides with the executive branch on national security matters. Experts say a GOP challenge would boil down to a question of whether the president’s constitutional powers to make and conduct war outweigh laws passed by the legislative branch.  

“It would set up a fascinating debate about the separation of powers,” said Cully Stimson, a former deputy assistant defense secretary for detainee affairs now with the Heritage Foundation. “Obama came into office as the anti-George W. Bush, he was the non-executive action guy. But as he grew into a wartime commander in chief, he employed many of the same legal arguments Bush did.”  

Experts say each side would lay out its arguments the following way: The White House would defend its actions as part of the president’s war-making powers under Article II of the Constitution while GOP lawmakers would point to Congress’ power of the purse and the so-called “Captures Clause” of the Constitution (though there is a scholarly debate over whether the latter focuses exclusively on property, not people).  

White House officials have been coy about a possible executive order to shutter the prison or transfer a large number of its detainees to “supermax” facilities on American soil.  

The president did not respond to a shouted question from a reporter about executive actions as he exited the Roosevelt Room. And on a morning call with reporters, two senior administration officials declined to directly address the matter.  

On Tuesday, White House Press Secretary Josh Earnest said he did not “want to take any of the president’s options off the table.” A day earlier, Earnest said Obama believes “working with Congress to succeed in closing the prison at Guantanamo Bay is the preferable outcome here.”  

To fulfill a promise he made on his second day in office, the president appears to little choice but to use his Article II war powers as commander in chief. After all, Senate Majority Leader Mitch McConnell, R-Ky., on Monday essentially declared the White House’s closure plan dead on arrival.  

“Since it includes bringing dangerous terrorists to facilities in U.S. communities, he should know that the bipartisan will of Congress has already been expressed against that proposal,” McConnell said Tuesday morning, referring to provisions in multiple defense policy bills that the president has signed into law.  

With Obama eager to burnish his legacy and his aides on the offensive during his final year in office, the president’s allies expect he will issue some kind of executive action before he leaves office.  

Such a case could find itself on a fast-track to the U.S. Supreme Court, said Lawrence Korb, a former Pentagon official now with the Center for American Progress, a Washington think tank with ties to the Obama administration.  

“I get the sense that the president is thinking about this a lot like healthcare or immigration,” Korb said.  

Senate Armed Services Chairman John McCain has said Obama should be careful about acting unilaterally. The Arizona Republican says his party would have no choice but to respond with a court challenge.  

Stimson said Congress has done all it can through legislation to keep the controversial facility open.  

“I don’t see what other legislative options they have,” Stimson said of lawmakers. “Something like a resolution of disapproval would be nonbinding — that would just be window dressing.”  

One former GOP leader has backed McCain’s call for litigation should Obama go it alone.  

“This is a case where the law says what the president cannot do,” former Senate Majority Leader Trent Lott, R-Miss., said late last year, referring to the years of congressional restrictions. “If he uses an executive order, I think McCain and others will take him to court. I think he’ll lose.  

“I think they’ve done what they need to do in legislation,” Lott said of Congress, noting Democrats have supported the Guantánamo limitations in ample numbers.  

Obama’s Democratic allies have long been wary of potential political backlash that could hurt the party.  

Although Senate Minority Leader Harry Reid, D-Nev., supports closing the facility in Cuba, he also said in 2009 that Democrats “will never allow terrorists to be released into the United States.”  

There is one more option, though a long shot: Congress could sit down with White House officials and hammer out a compromise Guantanamo plan.  

Despite tough talk from McConnell and other advocates of keeping Guantanamo open, Chris Anders, senior legislative council for the American Civil Liberties Union, told Roll Call he has heard whispers before about using year-end spending bills to settle the dispute once and for all.  

Congress likely will send Obama one more government-wide funding measure before he leaves office on Jan. 20.  

“We’ve heard they would like to work out how to undo that [defense policy law] language in [an] omnibus,” Anders said. “That’s the one piece of legislation the White House could still negotiate something with Congress on closing Gitmo.”