He shouldn't be allowed to express political views during the trial. It isn't a valid defense because his political views are completely unrelated to the matters at hand. The only purpose of the trial is to determine whether or not he did the things the government says he did. If he did them, then he broke existing laws.

If that were true, there would be no such thing as Jury Nullification. As long as the concept both exists and is valid, a defense that argues the law itself is in fact a completely valid defense.