hard times tend to breed political extremism in many forms. The question is how you use for a game? The most obvious use is to incorporate them as bad guys for the PCs to smack down but that's too easy. Some ideas
The players are tasked with taking down a Mexican drug cartel by raising and training a minuteman style militia to conduct raids over the border. Players of course have to deal with the implications of taking normal US citizens, training them into guerrilla warriors and then callously using them as pawns for their reasons.
A radical leftist, say an anti-WTO anarchist and a right wing extremist get together to assassinate powerful bankers or other corporate targets. Their blend of tactics and skills makes them far more dangerous than they were alone. Players have to get one killer to turn against the other.
Several extremists abduct and kill apparently random targets. When the players investigate, they found out disturbing biological anomalies in the victims' bodies. The extremists claim the victims were possessed by aliens...could they be right?