Breach of the Peace

Cynthia McKinney may have a pretty good case on Constitutional grounds:

... They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same...

As I understand it, the officer tried to stop her from entering a House office building, and she was only trying to defend her rights by slapping him silly.