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Members of the House of Representatives serve for two years. Senators serve for six years. There seats are automatically vacated at the end of their term. Maybe they can sit in the chair, but they have no rights as members of Congress. The more interesting question is what happens if the House or Senate refuses to seat new members as happened in the first session of the Thirty-Ninth Congress. The members of the new House determine the credentials of members, so it is always possible an election winner will not be seated.