By the late 19th century, the United States controlled vast territories yet to be incorporated as states into the Union. The prospect of disunion had ended with the Civil War, but the balance of partisan and sectional power in the Senate remained contentious with each new petition for statehood. In the post-war years, Congress was closely divided between Republicans and Democrats. From 1865 to 1900 Republicans controlled the Senate for all but four years; however, with the return of former Confederate states in the 1870s Democrats had a stronger hold on the House of Representatives. The admission of each new state prompted a partisan battle designed not to maintain a delicate balance but rather to secure a majority for years to come.
In 1886 representatives for the southern half of the Dakota Territory drafted a constitution and petitioned Congress to become a state separate from its northern neighbors. Members of both parties understood that South Dakota statehood would have an impact on the balance of parties in the Senate.
Republicans favored the proposal, but splitting the territory into two states met with Democratic opposition in both the House and the Senate. Republican senator and future president Benjamin Harrison took up the mantle for South Dakota admission. No doubt Senate Republicans saw admission of the new state as a means to bolster their majority status, but Harrison argued that "party advantage" should have no bearing on the decision to approve. To those who proposed balancing South Dakota, a state expected to lean Republican, by admitting New Mexico, which would lean Democratic, Harrison asked, "Can we not get rid of this old disreputable mating business? It grew out of slavery . . . but slavery has gone." The Republican-controlled Senate passed a South Dakota statehood bill in 1887, but the Democratic-controlled House did not act, leaving the territory waiting.
Democrats contended they would accept Dakota as a single state, but Republicans argued that it was simply too large to remain intact. When Senator Orville Platt, a Republican from Connecticut, brought South Dakota statehood to the floor again in 1888, he argued that not dividing Dakota would produce an "imperial state," a state of massive size and potential power. "We want equality," Platt declared, "physical equality as well as legal equality, among the States." While some senators worried that division of the territories into more states would make the Senate too large, Platt cautioned that a single Dakota could support a large population and lead to an "abnormally large representation in the House of Representatives."
Senate Democrats objected to splitting the Dakota Territory and were plain about the political reasons for doing so. Splitting Dakota into more than one state, Democratic senator Matthew Butler of South Carolina pointed out, would be "to seat upon this floor two Republican Senators, and settle, perhaps for some time to come, the question of political supremacy in this body." Orville Platt countered that the Senate had both a moral and a legal mandate to grant statehood to South Dakota, regardless of the Democrats' objections. Congress had discretion as to when to admit new states, Platt insisted, but "when the discretion of Congress is exercised upon any such ground as that two Republican Senators will occupy seats on this floor, it is no longer discretion; it is injustice and it is tyranny."
During the lame-duck session that began in December 1888, the House and Senate voted on an "omnibus" measure to admit North Dakota,, South Dakota, Montana, and Washington. The Democrats tried but could not secure inclusion of New Mexico. Democrats hoped that Montana would lean their way, but after a controversy in which the divided state legislature sent two competing pairs of senators—one Republican, one Democratic—the Senate accepted the credentials of the two Republicans in 1890. The other three states also elected Republicans, and each new state sent a Republican representative to the House. When the 51st Congress convened its first session on December 2, 1889, following the 1888 election of President Benjamin Harrison, Republicans controlled the House, the Senate, and the presidency. In 1890 Congressional Republicans also approved statehood for Wyoming and Idaho, securing their Senate majority for all but two years until 1913.
