President Obama’s chances of shuttering Guantanamo Bay through legislative action may be extinguished this week when the long-awaited defense authorization bill takes its final steps through Congress.

Capitol Hill is abuzz with anticipation that the president will soon submit a plan to close the detention facility in keeping with his 2008 campaign promise to shut it down. But time is running out as the GOP-led Senate prepares to vote Tuesday on the National Defense Authorization Act (NDAA), which contains language restricting transfers of Guantanamo prisoners to the U.S. mainland.

Earlier this year, the NDAA was considered a prime candidate to carry a plan to shutter Guantanamo — or at least force Congress to consider the issue. But Congress has effectively run out of time to shoehorn a plan into the NDAA —  meaning once the Senate approves the bill, Obama could have missed his last, best chance to close Guantanamo with congressional participation.

“They’re expecting us at the 11th hour, whenever it is, even after the defense authorization bill?” said Sen. John McCain (R-Ariz.), complaining that there’s no time to address closing Guantanamo in the NDAA. “I have to examine the plan, I have to show my colleagues that it’s something they can support,” McCain said. “You can’t just rubber-stamp it.”

McCain, who chairs the Senate Armed Services Committee, is one of the few outspoken Republicans who favor closing Guantanamo. He even worked language into the Senate’s original version of the NDAA that would have compelled lawmakers to vote on a plan to close Guantanamo once one was formally unveiled by Obama. But with no plan on hand, McCain’s language failed to make the final bill.

After six-and-a-half years of consultations with everyone from the president to Defense Secretary Ash Carter to senior White House advisers, McCain’s patience is wearing thin. Other Republicans worry the president is simply gearing up for an end-run around Congress by using executive action to shutter the facility, which is charged with holding terrorist suspects.

Last week, the president’s spokesman refused on various occasions to quell speculation that Obama may use his executive powers to close Guantanamo without Congress’ participation.

“That is a question for lawyers to answer,” White House spokesman Josh Earnest said on Thursday. “But as the president’s spokesman here, I’m certainly not going to take anything off the table in terms of him doing everything that he can to make progress on a national security priority that he’s identified.”

Earnest added that currently, however, the president “is focused on working with Congress to get this done.”

But Republicans in Congress don’t trust Obama’s intentions.

“I do think you’re going to see an executive action coming down to close down Guantanamo,” House Homeland Security Committee Chairman Michael McCaul (R-Texas), said on “Fox News Sunday.” “He’s done this before, and it’s hard to stop this kind of action.”

“I think this is outrageous and it’s egregious, it’s an overstep and an abrogation of the executive’s power,” said Sen. Pat Roberts (R-Kan.) in a news conference last week. “Why do we even have a Congress, if the president can issue an executive order on anything and in this particular case, endanger our national security? Then what are we even doing here?”

But the avenues to address Guantanamo through Congress are closing. And the current NDAA complicates the chances even further.

The legislation extends the ban on transferring detainees to U.S. facilities, and imposes additional restrictions on third-party transfers: under the measure, no detainees may be sent to Libya, Syria, Yemen or Somalia. Without the option of bringing prisoners into the U.S., the task of closing Guantanamo becomes difficult — there are still 112 detainees at the facility, only 53 of whom have been slated for transfer to foreign countries.

Obama could try to veto the NDAA, as he did last month with the first version of the bill, over a complaint that it relied too heavily on the government’s war chest to fund defense spending. Those concerns were addressed in the revised bill.

The White House hasn’t ruled out a veto as a possibility.

“We’ll have to take a look at exactly what passes Congress before making a determination about what the president will sign,” Earnest said last week. ‘This language that prevents the administration from effectively closing the prison at Guantanamo Bay is something that we have long opposed, and we continue to oppose it now.”

But at this point, any attempts to veto the new NDAA would amount to little more than a gesture. Last week, the House passed the latest version by a vote of 370 to 58, easily securing enough votes to override a veto. The Senate is also expected to secure a veto-proof majority on Tuesday.

While Congress could later consider a Guantanamo plan as a standalone bill, it’s unlikely that Republican leaders would allow something opposed by the large majority of their rank-and–file members to get floor time. And there is no other obvious, must-pass vehicle to which Guantanamo language could be attached, at least not one that is controlled by someone with the power or inclination to close the detention facility, like McCain.

“I don’t know. Honestly,” McCain said when asked this week if an omnibus appropriations bill could be another vehicle for a Guantanamo plan. And as for any other strategy, “It depends on what, how it’s accepted by colleagues.”

Most Republicans aren’t inclined to make any exceptions to the ban on transferring Guantanamo detainees to domestic soil. Some are downright livid at the administration for even exploring the option.

Sen. Cory Gardner (R-Colo.) charged last week that the Obama was “violating the law” by sending experts to survey a potential facility to house detainees in Colorado, “Despite the law making it very clear that money shall not be made available to assist in the transfer” of detainees, Gardner said.

He and other Republicans are also wary of any attempts to force a vote on the subject.

“I don’t know of anything else that’s coming besides the next vote on the NDAA, or the next iteration of it,” said Sen. Tim Scott (R-S.C.), who is also vocally opposed to the transfer of detainees to the U.S. and the closure of Guantanamo.

Congress has passed a defense authorization acts annually for the last 54 years, technically giving Obama one last chance to work in Guantanamo language before he leaves office. But in an election year, Scott added, “I think you’d be hard pressed” to close Guantanamo via the NDAA.

“The greatest fear we have is that the president will act on his own, violating or ignoring the laws,” Scott added.