The framers of the U.S. Constitution anticipated that their new government would endure into the future and incorporate new territories. In implementing the framers' vision, Thomas Jefferson called the United States an "empire for liberty," an expanding union of equal republican states knitted together by the bonds of interest and affection. Like much in the Constitution, however, the language setting the framework for the nation's expansion is vague, stating in Article IV that "New States may be admitted by the Congress into this Union." The only constitutional regulations provided by the framers insisted that no new states could be formed within or from territory of an existing state, or by combining states, without approval of the state legislatures and the U.S. Congress.
Even before the Constitution was debated and drafted, members of Congress under the Articles of Confederation planned for a union that would grow beyond the original colonies to incorporate new states. Land ordinances in 1784, 1785, and 1787 called for new western states to be admitted "on an equal footing with the original states." As delegates arrived at the Constitutional Convention, however, many worried about what the admission of new states meant for the strength of the Union and the existing balance of power in the federal government.
The Senate, with its equal representation of states, took center stage in the often contentious battles over admission of new states. Each additional state has held the potential to upset the existing balance of power in the Senate. In the decades before the Civil War, this meant that admission of new states threatened the very existence of the Union. After the Civil War, the threat of disunion abated, but the Senate still deadlocked over the acquisition of vast new territories that threatened to alter the Senate's regional and partisan balance. During the 20th century, the Senate considered statehood for long-held territories beyond the contiguous states. Throughout U.S. history, the Senate has overcome such impasses to expand the Union, bringing major changes to the United States and to the Senate.