LOL on the play one on Wikipedia comment An office under the United States is AFAIK any government office federal state or local I would imagine that just as it would be a breach of separation of powers for someone to be simultaneously Congressman and President it would be a breach of federalism for someone to simultaneously serve in two different governments I have no idea why the seniority date isn t the actual date of oath taking but he couldn t have voted on federal legislation until he resigned as a city councilman and was sworn into Congress so there was in reality no dual office holding