The majority leader is the floor leader of the largest caucus in a legislative body. Given the defacto two-party nature of the U.S. system, the majority leader is almost inevitably either a Republican or a Democrat, with their counterpart being of the opposite party. In bicameral legislatures, the majority leader in the lower house is often the second-most senior member of the majority caucus Speaker. The majority leader counterpart is the minority leader, whereas in the upper house the titular Speaker is often a separately-elected officer such as a lieutenant governor and the majority leader may in fact be the single most powerful member of the majority caucus.
The majority leader is often assisted in his role by whips, whose job is to enforce party discipline on votes deemed to be crucial by the party leadership and to ensure that members do not vote in a way not approved of by the party. Some votes are deemed to be so crucial as to lead to punitive measures (such as demotion from choice committee assignments) if the party line is violated; decisions such as these are often made by the minority leader in conjunction with other senior party leaders.