A politician has one main job while in office, and that is to get re-elected for another 2, 4, or 6 years depending on the length of their term. But that takes money.
Far more effort is spent on fundraising than actually legislating. In some cases, this is more than 3/4 of a politician's time or up to 8-10 hours a day. $18,000 is not an unusual sum for a congressional representative to have to bring in during every 24 hour period.
And where does the majority of this money come from? Bribery, which was made legal by the Supreme Court in Citizen's United v. the Federal Election Commission in 2010. Corporations and fat cats can spend all they want to ensure that their person wins, but then naturally they expect a payback. It's breathlessly obvious how it works.
There's a simple fix and that's Term Limits. You get one shot at office, you win, and then you can spend the entire term actually working for your constituents while speaking your mind without fear of getting tossed out.