There is no replacing them. When the election comes up my choice is the incumbent Republican or the Democrat. The Incumbents win in the primaries because of name recognition and ignorance amongst the electorate. This is why they are seldom replaced.
Also, my voting history is horrid. Four of the Reps who voted for the omnibus spending bill were people I voted for in the past. I thought they were all true Conservatives by the things they said....and did back then. I also voted for the Bushes and others who are showing their true colors now.
So how do we replace them when the candidates lie and tell us what we want to hear to get elected?
Which is why I'm pretty much at the point of not bothering to vote anymore. What's the point? Even if your guy wins, he'll just be "assimilated".