Here we examine the rise of Senate party caucuses. As we show, senators met in caucus only infrequently in the first half-century of the chamber’s history. The most notable use of early caucuses was by Republicans in the two chambers, who gathered on a quadrennial basis to nominate their presidential candidate. But, in 1841–45, led by the Whigs, the modern Senate party caucuses were born. In that brief period of time, senators began utilizing caucuses on a regular basis, both to conduct legislative business and to organize the membership of the Senate’s standing committees. In later years, first in 1856–62, then in the 1870s, Republicans and Democrats gave new structure and permanence to their caucuses. They created formal positions in the caucus, such as a regular caucus chairman, and caucus committees, including committees on committees, campaign committees, and ad hoc committees on the order of business. The emergence of the Senate caucus in 1841–45 coincided with the rise of a competitive two-party system, both in the electorate and in the Senate.