The Constituency
The citizens have a big effect on the bills that their representatives may introduce. The members of the House or Senate will introduce bills to help their constituency, to get publicity, or to say that they "did something" about an issue. These last two reasons are why bills usually die in committees. The citizens will call the staff of their Representative or Senator to have them help with an issue. Congressmen will have three different behaviors, one of them being representational, which means that the constituents vote for the person who has their best interests at heart. These people try to make their constituency happy by voting for the things that they want.