sabre rattling congressmen who keep doing local town halls where they tell their constituents "we lock the bad guys up forever"
That seems to imply that these same congressmen wouldn't be able to spin a 'US justice leads the world' story out of bringing terrorists to a fair trial? Maybe, but I'm still a little skeptical. Remember: some of these are the guys who convinced half the country that Iraq was behind 911. I think they would have no problem taking credit for solving a problem of injustice at long last, further "demonstrating that there is a real difference between us and the terrorists." And maybe blaming Obama for Gitmo being there in the first place.
If the US president can close Guantanamo over the objections of sabre rattling congressmen who keep doing local town halls where they tell their constituents "we lock the bad guys up forever", then it makes clear to those local folks that their congressman are actually pretty impotent.
There are plenty of "he wanted to close Guantanamo bay and let terrorists go, like Obama" commercials that aired during the election.
As the parent said, doing this to congressman, be they republican or blue dog democrat, would make him completely and totally ineffective in getting bills passed.
There is usually a broad difference between the posturing you hear about on TV, and the actual way they act in closed door meetings. In one case, they have to appear that way to keep their constituents happy. By and large, congressmen are not stupid (or at least, not egregiously so), they are just good at acting.
If you do something that is going to seriously threaten their apparent power position, however, it will no longer be acting.