The Senate Majority and Minority Leaders are two United States Senators who are elected by the partycaucuses that hold the majority and the minority respectively. These leaders serve as the chief Senate spokespeople for their parties and manage and schedule the legislative and executive business of the Senate. By rule, the Presiding Officer gives the Majority Leader priority in obtaining recognition to speak on the floor of the Senate. The Majority Leader customarily serves as the chief representative of his or her party in Senate, and sometimes even in all of Congress if the House of Representatives and thus the office of Speaker of the House is controlled by the opposition party.
The Assistant Majority and Minority Leaders of the United States Senate (commonly called Senate Majority and Minority Whips) are the second-ranking members of the party leadership of the United States Senate. The main function of the Majority and Minority Whips is to gather votes on major issues. Because he or she is the second ranking member of the Senate, if there is no floor leader present, the whip may become acting floor leader. Before 1969, the official titles were Majority Whip and Minority Whip.
Many state senates are organized in the same way as the United States Senate.
The Democrats began the practice of electing floor leaders in 1920 while they were in the minority. John Worth Kern (December 20, 1849 – August 17, 1917) was a Democratic United States Senator from Indiana. While the title was not official, he is considered to be the first Senate party leader (and in turn, the first Senate Democratic Leader), while serving concurrently as Chairman of the Senate Democratic Caucus. In 1925 the majority (at the time) Republicans also adopted this language when Charles Curtis became the first (official) Majority Leader, although his immediate predecessor Henry Cabot Lodge is considered the first (unofficial) Senate Majority Leader.
The Constitution designates the Vice President of the United States as President of the Senate. The Constitution also calls for a President pro tempore to serve as the leader of the body when the President of the Senate (the Vice President) is absent. In practice, neither the Vice President nor the President pro tempore—customarily the most senior (longest-serving) Senator in the majority party—actually presides over the Senate on a daily basis; that task is given to junior Senators of the majority party. Since the Vice President may be of a different party than the majority and is not a member subject to discipline, the rules of procedure of the Senate give the presiding officer very little power and none beyond the presiding role. For these reasons, it is the Majority Leader who, in practice, manages the Senate. This is in contrast to the House of Representatives where the elected Speaker of the House has a great deal of discretionary power and generally presides over votes on bills.
^No Republican whips were appointed from 1935 to 1944 since only 17 Republicans were in the Senate following the landslide reelection of President Franklin Roosevelt in 1936. Accordingly, the minutes of the Republican Conference for the period state: "On motion of Senator Hastings, duly seconded and carried, it was agreed that no Assistant Leader or Whip be elected but that the chairman be authorized to appoint Senators from time to time to assist him in taking charge of the interests of the minority." A note attached to the conference minutes added: "The chairman of the conference, Senator McNary, apparently appointed Senator Austin of Vermont as assistant leader in 1943 and 1944, until the conference adopted Rules of Organization." Source: Party Whips, via Senate.gov
^Democrats remained in control after November 25, 2002, despite a Republican majority resulting from Jim Talent's special election victory in Missouri. There was no reorganization as Senate was no longer in session. Party Division in the Senate, 1789-present, via Senate.gov