I've written a lot about the detention provisions in the National Defense Authorization Act restricting the ability of the executive branch to try suspected terrorists in civilian court and mandating military detention even in the case of domestic arrests. In their letter to the president, a group of Republican committee chairs lets the cat out of the bag as far as the long term objective of such policies:
We are concerned that the lack of a comprehensive military detention system will continue to have numerous detrimental results, including: incentivizing lethal operations over law of war detention; the loss of critical detainee-provided intelligence; forcing the United States to be wholly dependent on foreign governments to hold and provide access to detainees; and as in Warsame’s case, bringing terrorists to the United States.
We recognize that there are significant reasons for not bringing detainees to Afghanistan from other areas of ongoing hostilities. However, this is the very reason another location for detention must be used. Such a location is already available at Guantanamo Bay. The facilities at Guantanamo, which are state-of-the-art and the result of millions of U.S. taxpayer dollars, make it unnecessary to bring terrorists to the United States.
Republicans never tried to restrict the executive branch's authority to choose the forum in which suspected terrorists were tried prior to Obama taking office. To a certain extent, this is mere partisanship, but there's a larger goal at work here, which is not merely to prevent the administration from closing Gitmo but to reopen it permanently.
The letter lists a number of reasons for prioritizing law of war detention, but in reality there's only one, and it's not a very good one: preventing terror suspects from coming to U.S. soil. There are already hundreds of terrorists safely incapacitated in U.S. prisons, and there's nothing preventing intelligence officials from going back and questioning them if need be. Nothing has proved more effective in the long-term incapacitation of terrorists than the federal justice system.
This is an objection more political than policy-based -- "bringing terrorists to U.S. soil" is unpopular, in part because Republicans have broadly settled on the notion that terrorists can't be safely contained here, though they have no evidence for believing so. Without commenting on the wisdom of the administration's targeted killing policy, its conservative detractors have yet to convincingly argue that targeted killing takes place in lieu of detention, rather than as a result of the targets being unreachable by other means.
Given the broad continuity between Obama and Bush on national security policy, Gitmo is one of the few remaining points of divergence. As the most prominent symbol of the Bush era, his rehabilitation requires that it be kept open, so that history might judge both him and it more favorably.