The group is responding to a bill offered by Sen. Joe Lieberman (I-Conn.) and Rep. Charlie Dent (R-Pa.), which was itself a response to the killing of Anwar al-Awlaki, an American-born al Qaeda operative who helped recruit terrorists. Al-Awlaki's death in September led to a debate over whether the U.S. had the right to set up an operation to kill a U.S. citizen.

The bill, S. 1698, would solve that problem by revoking the citizenship of anyone who engages in hostilities against the U.S., or provides support for hostilities against the U.S.

But the Constitution Project argues that this change would violate the Constitution, and said the bill is similar to a 2010 proposal from Lieberman and Sen. Scott Brown (R-Mass.), who is also a co-sponsor of this year's bill. In 2010, the group stressed that under the Constitution, citizenship cannot be taken away simply because a person engages in hostilities against the U.S.

The group also argues that last year's and this year's bill could lead to the revoking of citizenship based not on a conviction for helping enemies of the U.S., but through an administrative determination that an offense has been committed. "[T]his legislation is seriously lacking in fundamental due-process protections because it would permit stripping of citizenship based on mere suspicion — not conviction — of terrorist activity," they said.