AUDIE CORNISH, HOST:

Closing the prison in Guantanamo Bay, Cuba, is a promise President Obama made it but has yet to fulfill. Today, the Pentagon sent Congress a long-awaited plan to shut it down. At the White House, the president made an ethical argument for doing so.

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BARACK OBAMA: Keeping this facility open is contrary to our values. It undermines our standing in the world. It is viewed as a stain on our broader record of upholding the highest standards of rule of law.

CORNISH: Joining me to talk about the plan and its prospects in the Republican-run Congress is NPR national security correspondent David Welna. Hey there, David.

DAVID WELNA, BYLINE: Hi, Audie.

CORNISH: Now, obviously this prison has been controversial ever since the first detainees arrived in Guantanamo more than 14 years ago. So at this point, how does the president propose to close it?

WELNA: Well, you know, I think his plan all along has been to reduce the prisoner population to the point where it gets hard to defend the amount of effort and expense needed to keep the place going. He's brought down the number of captives there to 91 by transferring scores of detainees to other countries. Thirty-five of those who remain are also eligible for transfer. That would leave only 56. And Obama's plan says they should be brought to the U.S. to be held here and, for some, possibly be tried by the federal court system.

CORNISH: Now, is there any indication where those detainees might be held in the U.S.?

WELNA: There is not, though the plan does say the Pentagon identified 13 unnamed potential facilities. But this was mainly to estimate how much it would cost to keep 30 to 60 detainees in the U.S. compared to the nearly half a billion dollars spent last year on the Guantanamo prison. The conclusion was that the U.S. would save up to $85 million a year housing those detainees at a single site on American soil, possibly one that would be built from scratch at a U.S. military base.

CORNISH: And you've been doing a lot of reporting on this, right? You're just got back from covering pretrial proceedings in Guantanamo of five alleged 9/11 plotters.

WELNA: Indeed.

CORNISH: Now, what happens to that case under this plan?

WELNA: It would likely continue here, though the president made clear today he thinks the military commissions process under which they're being tried has not worked out very well. He strongly prefers using the federal courts to try accused terrorists, as has been done successfully in hundreds of cases.

CORNISH: OK, so what's the response from Congress on this plan?

WELNA: Well, you know, it was actually Congress that demanded this plan for an alternate to Guantanamo. But it's clearly not what Republican congressional leaders want. They see this as an affront to prohibitions that they and even some Democrats have placed on transferring detainees to the U.S. or on spending any money on facilities to house them here, prohibitions that Obama himself signed into law. Here's Majority Leader Mitch McConnell today on the Senate floor.

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MITCH MCCONNELL: We'll review President Obama's plan, but 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.

WELNA: And House Speaker Paul Ryan put out a statement declaring we will not jeopardize our national security over a campaign promise.

CORNISH: All right, given all that, does it look like President Obama will be able to keep this pledge to shut down Guantanamo?

WELNA: You know, I think it would certainly mean, if Congress rejects this, that he could no longer count on Congress to help reach that goal. If lawmakers reject this plan, the question then arises would the president act on his own close Guantanamo? Some former members of his administration have argued that as commander in chief, Obama has all the authority he needs. Both his attorney general and defense secretary have said it would not be permitted under current law to bring detainees here. But the White House has not yet ruled anything out.

CORNISH: NPR's David Welna. David, thank you.

WELNA: You're welcome, Audie. Transcript provided by NPR, Copyright NPR.