Jonathan Capeheart has an amusing post on Mitt Romney's call for Abdelbaset al-Megrahi, the Lockerbie bomber, to be tried in the U.S., noting that Romney had previously called for military commissions for all suspected terrorists. Romney, as Zaid Jilani first noted at ThinkProgress, said that "We would try him here and see that justice is done."
I laughed when I first read Capheart's post. In fairness to Romney though, a military commissions trial for al-Megrahi would run into some serious ex-post-facto problems. The military commissions system was set up based on the Authorization to Use Military Force, which was passed by Congress more than a decade after the 1988 bombing of Pan-Am Flight 103. The Constitution of the United States explicitly says that "No Bill of Attainder or ex post facto Law shall be passed," which means you can't try someone based on a law passed after the offense allegedly took place. The commissions are intended for trials of people associated with al Qaeda and its affiliates based on the 9/11 attacks, and so it would be extremely difficult to argue that the Lockerbie bombing qualifies as a crime of war under legislation passed more than ten years later.
Granted, there are already some ex-post-facto issues with some detainees at Guantanamo already, but this is even more clear cut. Given that a military commissions trial for al-Megrahi would run head first into something explicitly prohibited by the Constitution, I don't think it's fair to attack Romney for "flip-flopping," particularly since if he had said he al-Megrahi should be tried by military commission liberals would be the first people pointing out that would be unconstitutional, and rightly so.
It's fair to point out that the Republican opposition to federal trials is ridiculous, but Romney isn't really flip-flopping here. Now if in response he retracts his remarks and says that he'd support trying al-Megrahi by military commission, that would be both flip-flopping and supporting a policy that is unconstitutional.
All of this is hypothetical since we probably won't get the chance to try him anyway.
UPDATE: I asked University of Texas Law School Professor Robert Chesney for his take, and he writes:
[T]he fact that this conduct predated the MCA is not necessarily dispositive. Several of the current commission's cases involved pre-2006 conduct, and the currently-pending Nashiri case (linked to the USS Cole bombing in 2000) predates 9/11--and there is a fierce battle underway as to whether the charges againt Nashiri are legit. But it's not so much a question of whether they are ex post facto, but rather whether there was a state of armed conflict/hostilities at the time of his original act. Put simply, there is no ex post facto problem with charging a run-of-the-mill war crime (e.g., killing civilians on purposes) so long as there was a state of armed conflict at the time of the action. But whether that was the case for Nashiri is a big debate, and it would be a very similar debate if commissions were attempted vis-à-vis the Lockerbie bombing.
So Romney could have made this argument if he wanted to, but given that al-Megrahi, unlike al-Nashiri, was not a member of al-Qaeda or an affiliated group it still seems like a big stretch to me.