and both of our senators voted for it

All I can find in the bill regarding the part of the article:
This dangerous compromise not only authorizes the president to seize and hold terrorists who have fought against our troops "during an armed conflict," it also allows him to seize anybody who has "purposefully and materially supported hostilities against the United States."
is this
http://thomas.loc.gov/cgi-bin/query/z?c109:S.3930:
Sec. 948c. Persons subject to military commissions

`Any alien unlawful enemy combatant engaged in hostilities or having supported hostilities against the United States is subject to trial by military commission as set forth in this chapter.
In context it makes sense, it should be defined though.
Note that the word alien is used in who is subject to this. Alien is defined as a noncitizen.