"If you want to run for Congress, you need money. A lot of money. To win election, U.S. House members raised, on average, $1,900 per day, every day, for an entire two-year term—including weekends and holidays. That’s $1.4 million per House member, on average. Winning senators raised $7.5 million, on average—$3,400 per day. (These figures are for the 2008 election, from the nonpartisan Center for Responsive Politics.)
"The main source of this campaign money is interest groups that want something from government. Once elected, politicians often pay back their campaign contributors with special access and favorable laws. This common practice is contrary to the public interest. MapLight's research tools help shine a light on the connection between money and votes, to help citizens hold legislators accountable.
"MapLight connects money and votes. We bring together, in one website, the money given to politicians with each politician's votes. We provide data, research services, and online tools that work together to make patterns of money and influence more transparent. Connections between campaign contributions, interest groups and votes that would have required days or weeks of manual research are now available at the click of a mouse."
http://maplight.org/us-congress/guide