Can the House refuse to seat a winner from the other party?

On March 1, 1967 the House voted 307 to 116 to exclude Adam Clayton Powell Jr. of New York even though a select committee found he met the constitutional qualifications. In 1969 the Supreme Court held the exclusion unconstitutional, because in judging the qualifications of its members Congress is limited to the standing qualifications prescribed in the Constitution. The Court also held that a refusal to seat can be reviewed in court, since deciding what the Constitution's qualifications mean is a judicial task.

What the House keeps under Article I, Section 5 is the power to judge whether a member-elect actually won the election and holds a valid return, and whether the person meets the constitutional qualifications. Removing someone who is already seated is a different act, expulsion, and it takes a two-thirds vote.