https://en.wikipedia.org/wiki/Unseated_members_of_the_United_States_Congress
Both houses of the United States Congress have refused to seat new members based on Article I, Section 5 of the United States Constitution which states that:
"Each House shall be the judge of the elections, returns and qualifications of its own members, and a majority of each shall constitute a quorum to do business; but a smaller number may adjourn from day to day, and may be authorized to compel the attendance of absent members, in such manner, and under such penalties as each House may provide."
This had been interpreted that members of the House of Representatives and of the Senate could refuse to recognize the election or appointment of a new representative or senator for any reason, often political heterodoxy or criminal record.
However, the U.S. Supreme Court, in Powell v. McCormack (1969), limited the powers of the Congress to refuse to seat an elected member to when the individual does not meet the specific constitutional requirements of age, citizenship or residency. From the decision by Chief Justice Earl Warren: "Therefore, we hold that, since Adam Clayton Powell Jr., was duly elected by the voters of the 18th Congressional District of New York and was not ineligible to serve under any provision of the Constitution,
the House was without power to exclude him from its membership."
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