The biggest legal hurdle facing the authorities–who believe that the young men were part of a Qaeda sleeper cell in Lackawanna, N.Y.–is a two-year-old ruling that a critical part of the 1996 antiterrorism law under which they have been charged is unconstitutionally vague. That was the view of a federal appeals court in an unrelated California case.

Another likely defense: a freedom-of-association claim echoing those raised by communist defendants back in the 1950s. The issue then was whether people could be prosecuted for being members of–or associating with–the U.S. Communist Party without proof of specific intent to further its illegal goal of violent revolution. The issue now is whether people can be prosecuted for providing “material support” to a foreign terrorist group without proof of specific intent to further its illegal terrorist activities.

Defense lawyers claim that the six had had no terrorist intent and had wanted only religious training when they traveled to Pakistan and Afghanistan in the spring and summer of 2001. And “our law generally requires that some kind of individual culpability be proven” to warrant criminal punishment, says civil-liberties lawyer and Georgetown law professor David Cole, author of a new book entitled “Terrorism and the Constitution.”

The Justice Department may or may not be able to prove terrorist intent in this case. Its argument, in any event, is that it need not shoulder that burden. The department said the same in the now-settled case of American Taliban supporter John Walker Lindh, who was also charged under the “material support” law (among others). Indeed, the government claims the power to prosecute even people who raise money for the humanitarian activities of groups like Hamas, which is both an officially designated terrorist organization and a major provider of social services to Palestinians.

In the Antiterrorism and Effective Death Penalty Act of 1996, Congress made it a crime to provide a designated terrorist group with “material support or resources,” no matter what the defendant’s intent. “Material support” is defined to include money, “lodging, training, safehouses, false documentation or identification, communications, equipment, facilities, weapons, lethal substances, explosives, personnel, transportation and other physical assets, except medicine or religious materials.”

The Lackawanna defendants appear to have provided Al Qaeda nothing except “personnel”–themselves–and may say they did not do that willingly. (It would be a stretch to say that trainees were “providing” training.) This is where the federal appeals-court decision becomes relevant: it struck both “personnel” and “training” out of Congress’s definition of “material support,” ruling these critical terms unconstitutionally vague because they do not give fair notice of what kind of conduct is illegal.

The trouble with “personnel,” the three-judge panel reasoned, was that “it blurs the line between protected expression and unprotected conduct,” leaving it unclear whether one could be prosecuted merely for advocating a terrorist group’s cause. The problem with “training” was that it could be read as including such innocuous activities as “teaching international law” to members of terrorist groups.

The Justice Department is appealing that decision, and some judges, including Judge T. S. Ellis III in the Lindh case, have disagreed with the appeals court and rejected the vagueness argument.

Defense lawyers will also make a broader freedom-of-association attack on the entire “material support” provision, now punishable by up to 15 years in prison, by claiming that unless prosecutors prove specific intent to support terrorist activities, it would violate the First Amendment to punish people for providing money or other material support to a group involved in legitimate activities as well as in terrorism.

This argument relies on Supreme Court rulings that people cannot constitutionally be punished for mere membership in, or association with, the Communist Party or other subversive groups. The government must also prove, in the words of a 1982 decision, “that the group itself possessed unlawful goals and that the individual held a specific intent to further those illegal aims.”

But the same appeals court that struck out the words “personnel” and “training” upheld the rest of the “material support” provision. While the First Amendment protects freedom of association and advocacy, it ruled, material support is far different, because even if “intended to aid an organization’s peaceful activities,” it “frees up resources that can be used for terrorist acts” and for buying “weapons and explosives.” The judge in the Lindh case agreed.

Prosecutors could also argue that Al Qaeda–unlike the Communist Party and Hamas–is a purely criminal organization with no legitimate activities at all.

What’s the outlook? It seems unlikely that in these times the Supreme Court would require proof of specific terrorist intent. And don’t bet on the justices upholding the appeals court’s objection that “personnel” and “training” are too vague.

After all, that decision came during the pre-9-11 era, when Al Qaeda’s terrorist training camps were far from judges’ minds. It would clearly be constitutional for Congress to write a law specifically making it a crime to willingly accept military or terrorist training from any designated terrorist group. And although Congress has not written the law quite that way, the justices may prefer a little bit of vagueness to freedom for Al Qaeda trainees.