Some people try to boil down the essence of Congress into one single factor - money or 'follow the money.' To me however, you can boil Congress down very easily by examining its leadership. Unlike the Executive Branch, where leadership beyond the President can be diffuse, the Congress lives and breathes on its leadership structure. Really savvy Congressional mavens keenly follow the leadership's movements and target their efforts almost exclusively on the leadership.
Congress has a host of varying leadership positions ranging form the almost imperial Speaker of the House to the odd role of the President of the Senate. In most cases the positions revolve around the two party conferences. Mirror leadership positions exist for both the Majority party and the Minority party.
The Speaker of the House is one of the two leadership positions created by the Constitution; all the others are creatures of the Congressional rules over the past 200 or so years. The Speaker, whose obvious origins lie in the Speaker of the House of Commons in London, has a distinctly American twist. He doesn't (nor to my knowledge has ever) worn a wig, and even though his primary role is to preside over debate in the House of Representatives, I rarely have seen a Speaker actually do so. He usually passes off that tedious task to a rotating list of members from the majority party. The Speaker of the House is the presiding officer of the House and usually also assumes the ceremonial roles as the official representative of the House. He manages all administrative functions and all debate, correspondence and even requests for sick leave. The Speaker is also third in the line of succession to the Presidency. If something simultaneously happened to both the President and the Vice President, the Speaker of the House would become President. The Speaker is also a voting member of the House. The current Speaker Dennis Hastert is a member of Congress for a suburban Chicago district. The Speaker of the House is as powerful as he chooses, though the days of absolute rule, are long gone. In an earlier era, the Speaker of the House reserved the role of Chairman of the Rules committee for himself which effectively sowed up all real power in the House into one person. Modern Speakers have delegated the Rules Committee Chairmanship to trusted colleagues.
The presiding officer of the Senate is hardly worth writing about. The President of the Senate is also known as the Vice President of the United States. His position in the Senate is also provided for in the Constitution and it is the only position that is selected by the Constitution. All the other positions involve some form of election, either by the whole chamber or the party conferences. The President of the Senate has no vote except in case of a tie and I have never heard the President of the Senate ever make a speech on an issue before the Senate. He, like the Speaker of the House, also rarely manages debate and on most occasions hands over the daily debate management to a rotating list of members of the majority party. There are countless anecdotes of Vice Presidents lingering in powerless boredom in the Senate. The House functions under the authority of the Speaker, the Senate pays simple ceremonial homage to its President and, unless there is a tie vote, could care less about its presiding officer. The Speaker normally is present somewhere in the Capitol building when Congress is in session; the President of the Senate (the Vice President) is rarely in the building.
Below the busy Speaker of the House and the bored President of the Senate is a line organization of party leadership that, when it functions effectively, can look more like a disciplined Army. The rules of the chambers, however, make the party leaders in both chambers very different.
The majority leader of the House along with the Speaker controls what will and will not be addressed on the floor. They hold almost absolute control over how legislation will proceed. The Majority leader has numerous deputies and a cadre of interestingly named "whips" to push, cajole and harass party members to toe the party line on all legislation brought to the floor. A member who annoys the whip and the majority leader will have some hard times on his own legislation and committee assignments in the future. The Minority Leader has a similar organization, although since they do not have the votes to organize the chamber they bide their time in powerlessness. They do take advantage of any opportunity to press their opposition and embarrass the majority leadership, however.
The majority leader in the Senate similarly holds the cards on the operation of the Senate but is severely constrained by the collegial rules of the Senate. Minority rights are enshrined in the rules of the Senate to the point that there is time you wonder who really controls the chamber, especially if the division of the parties is close. There are few minority privileges in the House of Representatives. Like the House, the Senate also has a whipping organization to muster support for legislation on the floor.
Beyond the Floor leaders and whips, the real influential power lies, in the committees, especially in regards to writing legislation. Committee Chairpersons, particularly in the House, are by far the most powerful people in Washington, DC, for their particular domain. They hold control of the committee and since the majority party assures that there is always more of them vs. the other party on the committee, he or she wield almost complete control over committee's operation. Prior to party rule changes in the 1980's, committee chairman ruled their fiefdoms for decades since seniority dictated they continue to hold the chairmanship for as long as they were in office.
During the hundred or so years of Democratic Party control of the South after the Civil War, Southern Congressmen usually held the role of Committee Chairman since they were rarely turned out of office by the voters and usually left the House or Senate in death. The new Republican majority in the 1980's introduced term limits on chairmanships and the South no longer is solidly Democratic, so there is more frequent turnover than before.
A committee chairman determines what is brought forward in committee and usually has overwhelming influence on the language of measures that come out of the committee. Most lobbyists are keen to get the "chairman's mark" (or the committee chairman's version of the bill) prior to a committee markup. They know that will be closer to the final version than the original bill.
Some Committees are more powerful than others. In the House, the Rules Committee determines the exact process of how a measure will be debated on the floor, it determines the time, and even the amendments (if any are allowed) that can be offered. The Senate has a similar committee but it does not play a similar role in controlling floor debate as in the House. If you are kowtowing to a rules committee chairman, make sure it is in the House not the Senate. Other House Committees of importance are determined by the field which they cover. For example, Ways and Means addresses all tax and revenue measures and is therefore very important. This same function in the Senate is split between several committees. Appropriations committees in both chambers address the spending of the government's revenues and, of course, are keenly courted as a result.
The Senate has some additional functions that the House does not have, though the House frequently pretends to have this authority. The Senate must confirm Presidential appointments, so the Senate committees have significant leverage on Executive agencies. They also control military commissions, judicial appointments, and treaties. As a result, Committee chairmen most frequently in the news are usually Senate chairpersons - particularly Senate Judiciary and Senate Foreign Relations Committee Chairpersons.
Beyond Committee chairpersons are party positions. Each party in each chamber has a Campaign Committee, with chairpersons who dispense money to needy campaigns and enforce fundraising discipline on money-shy members.
Congressional leadership, especially people who hold positions for a long period of time, usually extend their influence throughout the city by staff patronage. Seasoned staffers and protégés are placed throughout the city, in lobby firms, Federal agencies, and even the courts.
There are no more persistently powerful positions in Washington, DC, than a leadership position in Congress. Depending on the issue and the polls, any one of them could be more influential than the President. Their tenure in the past has often been secured for life (or near life, with good behavior) and the potential to expand and entrench influence through legislative measures, campaign cash, and staff protégés further solidifies these roles. Congressional leadership can wane, however, and there are factors that can diminish their power. If a majority party has a razor thin majority, party discipline can be difficult. A scandal or hint of scandal could also reduce a leader's edge. And of course, the voters could decide to not reelect a leader, an event that has happened surprisingly in recent years.
If you are following an issue or a bill in Congress, always check to see where the issue lies on the majority leadership agenda and check on the appropriate committee chairperson's position. The answers to these positions will provide the primary determination as to the issue or bill's success in Congress.
The most wrenching event for Congressional leadership is when the majority party of a chamber becomes the minority party - it is the most dynamic and potent event in the life of a Congress. The all powerful become mere Members of Congress and Senators - everything changes. Sometimes it's payback time for old grudges and slights. It doesn't happen often, but the upcoming (November 7) elections may usher in such a traumatic event once again.