Ratification of the U.S. Constitution in 1788 led to a transition from the Congress under the Articles of Confederation to a new federal government. On September 13, 1788, that soon-to-expire Congress issued an ordinance giving states the authority to conduct elections for their senators and representatives. On September 30, 1788, Pennsylvania became the first state to elect its two senators. On March 4, 1789, the first group of elected senators reported for duty.

From 1789 to 1913, when the Seventeenth Amendment to the U.S. Constitution was ratified, senators were elected by state legislatures. Beginning with the 1914 general election, all U.S. senators have been chosen by direct popular election. The Seventeenth Amendment also provided for the appointment of senators to fill vacancies.

There have been many landmark contests, such as the election of Hiram Revels, the first African American senator, in 1870. In 1932 Hattie Caraway of Arkansas became the first woman elected to the Senate. In 1948, when Margaret Chase Smith was elected to the Senate, she became the first woman to serve in both houses of Congress. In her 1960 bid for reelection, Smith made history again when she defeated Lucia Cormier. Senators have been elected by write-in votes and some have seen their elections contested. There have been landslide elections, while others were decided by the slimmest of margins.

Unusual circumstances also have produced some uniquely historic elections. In 1858, for example, the new Republican Party gained ground, setting the stage for 1860, when Republicans took control of the Senate, the House, and the presidency. In 1861, in the midst of the Civil War, a pro-Union faction within the seceded state of Virginia created its own government and elected its own U.S. senators.

Since the first elections in 1788, 1,950 men and women have served in the United States Senate.