Why was the house of representatives involved in the election of 1800

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Elections to the United States House of Representatives for the 7th Congress in 1800 and 1801, at the same time as the 1800 presidential election, in which Vice President Thomas Jefferson, a Democratic-Republican, defeated incumbent President John Adams, a Federalist."

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https://en.wikipedia.org/wiki/United_States_House_of_Representatives_elections,_1800

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