The United States Congress is the seat of the national legislature of the United States of America. It includes the House of Representatives and the Senate.
During the CIA's covert operation in Russia, Charles Xavier telepathically probes Emma Frost and discovers Sebastian Shaw's intentions of igniting WWIII in order to wipe out humanity and insure mutant supremacy. In a scene depicting a hypothetical future, the Hellfire Club is seen presiding over a torch-wielding crowd of mutants following the end of the war, and depicts the U.S. Capitol in smoldering ruins.
As Jean Grey concludes a presentation to the U.S. Senate on the specifics of mutation and the X-Gene, Senator Kelly engages in a debate with her, attempting to highlight the alleged danger mutants pose to society. He ultimately advocates the Mutant Registration Act so that, in his own words, the public can "...know who they are, and above all, what they can do!"