As surely as water flows downhill, money in politics flows to where the power is. Individuals and interest groups will give campaign contributions to politicians in the best position to deliver what they're looking for. That means incumbents get vastly more than challengers, committee chairmen and legislative leaders get more than rank-and-file members, and parties in power get more than parties in the minority.

Incumbents vs. challengers

Interest groups like safe bets, and in the political world nothing is so safe as giving money to a politician who's already in office. Re-election rates are so high that nearly all interest groups give an overwhelming majority of their campaign dollars to incumbents. They typically reserve a small part of their budgets for promising candidates in open seat races where no incumbent is running -- and very little, if anything, for challengers seeking to oust the current officeholders. Labor unions are more likely to take a chance on a new candidate than business groups, but even they put the great majority of their dollars into the re-election campaigns of incumbents.

Congressional committees

In the halls of the Capitol in Washington, D.C., most of the nitty-gritty work of crafting new laws gets done not on the floor of the House or Senate, but at the committee level. No important bill is considered without first winning approval from at least one committee, and often several. Those panels are thus enormously important to the industries they oversee. Because of that, expect members of the Agriculture Committee to get the most in contributions from agribusiness interests; Banking Committee members from financial firms, etc. And expect the chairmen of the committees -- and ranking minority members -- to top the list of recipients. When a member of Congress changes committees, the main source of his or her financial support typically changes as well.

Majority vs. minority party

When the balance of power changes on Capitol Hill, the contributions from interest groups shift almost immediately to the party in control. It's so predictable you can actually see the shift in a graph. Even interest groups that predominately give to one party or the other -- such as labor unions giving to Democrats -- often shift at least some of their money to newly powerful members of the other party. For interest groups that have always been generous to both sides, a shift in party control on Capitol Hill usually means the majority of their dollars will now go to the side in power.