Some of the most difficult election disputes involved senators appointed by the governors of their states to fill vacancies while the state legislatures were out of session. The seemingly straightforward language of the Constitution, "if vacancies happen...during the recess of the Legislature of any State, the Executive thereof may make temporary appointments until the next meeting of the Legislature, which shall then fill such vacancies" (Article I, section 3), turned out in practice to be far from simple when political conflicts arose. The ambiguity of the rules left the actual practice open to the influence of politics. Opponents of a particular individual or party--either in the home state or within the Senate--could bring a procedural challenge that was often decided by a party-line vote.
Before 1913 the appointment lasted only until the next meeting of the legislature. Under direct election, the appointment typically lasts until a special election can be held to fill the remainder of the term. Prior to direct election, questions often arose when a legislature met and failed to elect a candidate before the commencement of a Senate term. On such occasions, the state's governor frequently appointed an individual to serve until the legislature could succeed in completing an election, so that the state would not be left without full senatorial representation. Between 1794 and 1893, the Senate reviewed at least nine such cases, seating all but two of the claimants. In 1893, the Senate reversed its previous practice and refused to seat three western senators, in part from concern about how the new members might vote on legislation pending at the time. The Senate then followed the precedent of those cases in denying admission to two other appointed senators in the 1890s.
The advent of direct election ended this particular dilemma, but questions regarding appointments continued to arise for several years, because the Seventeenth Amendment states that "the legislature of any state may empower the executive thereof to make temporary appointments." Challenges to several appointed senators hinged on whether the state's legislature had in fact adopted a measure bestowing the power on the governor. After 1926, the only challenges to appointments related to conditions in the specific case, such as the appointee's qualifications, when an appointment should terminate, or whether an incoming or outgoing governor had the right to make the appointment.
Source: Adapted from Anne M. Butler and Wendy Wolff. United States Senate Election, Expulsion, and Censure Cases, 1793-1990. S. Doc. 103-33. Washington, GPO, 1995.