Chris Anders of the ACLU Washington Legislative Office wrote

The Senate is going to vote on whether Congress will give this president — and every future president — the power to order the military to pick up and imprison without charge or trail civilians anywhere in the world. The power is so broad that even U.S. citizens could be swept up by the military and the military could be used far from any battlefield, even within the United States itself.

According to the ACLU, the NDAA in its current form

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(1) Explicitly authorizes the federal government to indefinitely imprison without charge or trial American citizens and others picked up inside and outside the United States;

(2) Mandates military detention of some civilians who would otherwise be outside of military control, including civilians picked up within the United States itself; and

(3) Transfers to the Department of Defense core prosecutorial, investigative, law enforcement, penal, and custodial authority and responsibility now held by the Department of Justice.