"Frist's seat is open, and Santorum will probably have a tough challenge. Who else?"
Well, if you want to call Lincoln Chafee a Republican, then he's on top of the list---I don't think he will survive in 2006, and I would rather have him switch to the Democrats to allow us to (i) have a real Republican instead on his three committees and (ii) run a solid Repblican against Chafee instead of against Congressman Langevin in the general election (we can beat Chafee, but not Langevin). If Governor Mark Warner runs against Senator George Allen, Allen will have a tough race. I think Senator Burns of MT, who was reelected with like 52% in 2000, will get an easier ride in 2006, since the guy who did so well against him just got elected governor and the Dems don't have a very deep bench in MT. Other than that, I think the only seats we will have to defend are possible open seats in TX and OH (if Hutchison and DeWine run for governor) and MS and IN (if Lott and Lugar retire). The best action in 2006 will come from all of those vulnerable Democrats trying to keep their seats, especially if we have a few Dem retirements, which I think are likely---I think the GOP will pick up 2-4 net Senate seats in 2006.