I believe that part of the explanation for the increase is that politics is more and more a high-stakes business. The parties are reluctant to swap out a known

I believe that part of the explanation for the increase is that politics is more and more a high-stakes business. The parties are reluctant to swap out a known winner and risk losing a seat. I admit that this does not account for cases such as Strom Thurmond or Orrin Hatch on the one side or Dianne Feinstein on the other.