Article I, Section 5, Clause 1:
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.
Each House, in judging of elections under this clause, acts as a judicial tribunal, with like power to compel attendance of witnesses. In the exercise of its discretion, it may issue a warrant for the arrest of a witness to procure his testimony, without previous subpoena, if there is good reason to believe that otherwise such witness would not be forthcoming. 1 It may punish perjury committed in testifying before a notary public upon a contested election. 2 The power to judge elections extends to an investigation of expenditures made to influence nominations at a primary election. 3 Refusal to permit a person presenting credentials in due form to take the oath of office does not oust the jurisdiction of the Senate to inquire into the legality of the election. 4 Nor does such refusal unlawfully deprive the state that elected such person of its equal suffrage in the Senate. 5