Basically a congressman or woman needs funding to get elected. Without TV ads and lots of exposure, US citizens do not often take much notice of a politician. In order to get this money they need to look to big business with big money. That often means that a politician needs to look favourable in order to gain this money and will do what big business says, rather than what their votes say.
The 2010 house elections cost $1 billion. The 2010 Senate elections cost $759 million. This is the at least cost, there is a lot more than this.
Where did this money come from?
For example Maurice Hinchey, congressman for NY Lockheed Martin $1,000, American Airlines, $1,000, DCCC made 5 lots of $5,000, where did this money come from in the first place? Honeywell international $5,000.
Let's take the winner in the North Dakato Senate race, John Hoeven, who got over $1 million from corporate sources. A guy who made minus $2,000 from Newfield Pac. He got money from all over the place, Washington, Dallas, LA, Anchorage, $1.2 million from companies like this. But he got $2 million from individuals. Some of these individuals come from places like Denver, Dallas. Workers for TRT holdings put quite a lot of money in, a Texas based oil company. The CEO put $1 million into American Crossroads that gives money to republican candidates, the workers are putting money towards candidates in other states on the other side of the country. So of this money that comes from "individuals", a lot comes from corporations that have their own agendas too. One look at one page from the FEC pages shows us that 8 people from ND paid money (all $2,400 on this one page), one person from PA, two from OK, two from TX, two from OK, two from CA, two from AZ, two from MN and one from NY. So 14 from outside the state, 8 from the state.
All seems a little fishy to me.
http://action.change-congress.org/pages/fair-elections-now-act
"Special interests pump millions of dollars into our elections, giving them more influence over the political system than anyone. Until we fix our broken campaign finance laws, these special interests will block substantive reform, from the left and from the right, on issue after issue."
http://www.newscientist.com/blogs/thesword/2010/01/corporate-funding-of-election.html
"A victory for free speech, or a reckless capitulation to special interests with bulging wallets?
That's the new political debate in the US, following Thursday's ruling by the Supreme Court, which loosens restrictions on corporate spending on advertisements to endorse or oppose particular political candidates."

