Legislation is difficult to pass in the US Congress because the system was designed to enact slow meaningful change rather than an immediate response. This is done through a certain level of complication where as a piece of legislation is submitted to a gauntlet of obstacles that could prove fatal to the progression of the bill. These obstacles include various veto powers and a complex power structure that is the result of the formation of various committees and sub-committees that have over-site over the various issues that come before the public.
The congressional leadership holds the power of assigning certain members of congress to political subcommittees that oversee and possess over-site over the various issues that are addressed by the government. The positions of power among these subcommittees are coveted, as they bring power and control to those who are placed in power by the congressional leadership. As such, the congressional leadership is empowered by the influence they hold over the appointment process.
The US House of Representatives is headed up the Speaker of the House, followed by the House Majority and Minority leadership. The Speaker is the central hub of leadership, while the Majority and Minority Leadership is responsible for rallying their respective interests.
The US Senate is constitutionally headed up by the Vice President of the United States and the President Pro Tempore, who is usually the senior most member of the senate. The leadership is also composed of various positions of power among the political parties such as the majority / minority leaders, and the various political whips responsible with ensuring the members behave along party lines.