South dakota constitution
Article XXVI: schedule and ordinance
Section 14

Immediately after the organization of the Legislature and taking the oath of office by the state officers, the Legislature shall then and there proceed to the election of two senators of the United States for the state of South Dakota, in the mode and manner provided by the laws of Congress for the election of United States senators. And the Governor and the secretary of the state of South Dakota shall certify the election of the said senators and two representatives in Congress, in the manner required by law