Does Joe Lieberman's Terrorist Expatriation Act accomplish anything other than positioning him to the right of Glenn Beck and John Boehner? In the unlikely event that Lieberman succeeds in adding "material support" for terrorism to the list of actions that can result in loss of citizenship, the practical consequences would depend on how the courts read the intent requirement of the law that Lieberman wants to amend.
In the 1967 case Afroyim v. Rusk, the Supreme Court ruled that Congress has no authority to revoke an American's citizenship unless he voluntarily renounces it. Thirteen years later, in Vance v. Terrazas, the Court said that requirement is not satisfied simply by showing that someone voluntarily committed one of the "expatriating acts" listed in the Immigration and Nationality Act (which include treason, taking an oath of allegiance to a foreign state, and serving in the armed forces of a hostile country). The government also has to provide evidence that he thereby intended to renounce his citizenship—a requirement that Congress later added to the law. At the same time, the Court upheld the "preponderance of evidence" standard specified by Congress: If the State Department decides to take away someone's citizenship based on an expatriating act and he challenges that decision in court, the government must show it's more likely than not that he 1) voluntarily committed the act 2) "with the intention of relinquishing United States nationality."
But this adversarial process would make no sense if the appellant could win simply by disavowing any such intent. In Vance the Court made it clear that the government could prove the "assent" of someone without his consent (emphasis added): "It is difficult to understand that 'assent' to loss of citizenship would mean anything less than an intent to relinquish citizenship, whether the intent is expressed in words or is found as a fair inference from proved conduct." It's not clear exactly what that would mean in the context of "material support" for terrorism, a very broad category of behavior that encompasses pure speech advocating lawful activity. But presumably the government would argue that the nature of the alleged act suggested an intent to give up citizenship.
Writing in The Washington Post, Georgetown University law professor David Cole notes the First Amendment implications of the "material support" statute, a law "so broad that it makes it a crime to file an amicus brief in the Supreme Court, to lobby Congress, to teach human rights or to write an op-ed piece, so long as it is done with or for a designated group." But he argues that proving an "intention of relinquishing United States nationality" would be an insurmountable barrier in practice. "Only those who voluntarily, knowingly and intentionally renounce their citizenship will lose it," he says.
Cole also suggests that taking away an American's citizenship based on alleged ties to terrorism would not provide the end run around due process that Lieberman wants. While the law authorizing military tribunals for terrorism suspects applies only to noncitizens, he says, there is no constitutional basis for that distinction. "There is no legitimate justification for selectively subjecting noncitizens to substandard criminal process," Cole writes. "Citizen terrorists pose as much a threat as do noncitizen terrorists, and both are guaranteed the same constitutional rights in the criminal process, military or otherwise."