Some members of congress receive money and/or other favors (bribes) from special interest groups to vote for or against a bill that directly affects the special interest group. Even worse, some members of congress accept bribes from both special interest groups that support and don't support a particular bill. Often these bribes are considered "campaign contributions".
Bribery is defined as "the offering, giving, receiving, or soliciting of something of value for the purpose of influencing the action of an official in the discharge of his or her public or legal duties" and is a crime.
Members of congress should be held accountable, just as citizens are held accountable, for bribery.