In the wake of Jeb Bush’s surprising announcement today that he will not run for Senate in Florida in 2010, this seems like an appropriate time for the monthly refresh of our ridiculously premature 2010 Senate Rankings.
To review the basic ground rule: the races are ranked in terms of their likelihood of changing parties. In some cases, the ranking is the result of an incumbent being vulnerable, and in other cases, because the incumbent may retire — we try our best to mesh these things together into some inkling of probability. Away we go:
1. Florida (R-Open) 
Without Bush in the race, the field is wide open on both sides. Given an open seat and one of the purplest purple states in the country, that means this race meets the very definition of a toss-up.
2. Nevada (D-Reid) 
No, the Republicans don’t really have a credible opponent yet, but one thing the past few weeks have made clear is that Reid is going to be a focal point for criticism — from both the right and the left — and there’s a strong likelihood that someone will come out of the woodwork. I also think people may be underrating the chances that Reid gets sick of it all and retires.
3. Pennsylvania (R-Specter)
Looking more likely that Specter may face a primary challenge, but the general election remains his central concern. The Chris Mathews rumors have cooled off some, but it’s not clear whether that’s good or bad news for Specter.
4. Ohio (R-Voinovich)
5. Kentucky (R-Bunning)
6. Kansas (R-Open) 
Remains an overlooked race; if Kathleen Sebelius wants the seat, she probably has a leg up. But there are rumors that she’ll become chancellor at the University of Kansas rather than running for Senate.
7. Missouri (R-Bond)
8. Illinois (D-??) 
I’m not sure what other possible permutation of circumstances could do more to make a generally safely blue seat competitive. Still, the Democratic bench is fairly strong, and they’re lucky this is all taking place in Illinois instead of, say, Ohio.
9. Colorado (D-Bennet) 
I was prepared to move this race down when it looked like Denver Mayor John Hickenlooper was going to be Ken Salazar’s replacement, but with unknown schools superintendent Michael Bennet getting the nod instead, it becomes a much more attractive target for Republicans. Still, the state is turning bluer, and most of their candidates haven’t polled very well against nearly any Democrat. They may also wind up punting on this race if they make the mistake of nominating Tom Tancredo.
10. North Carolina (R-Burr)
11. New Hampshire (R-Gregg)
12. Texas (R-Open?) 
It appears nearly certain that Kay Bailey Hutchinson will vacate her seat to run for Governor, and now the Republicans will have a formidable opponent, as moderate Dem and Houston Mayor Bill White has declared his interest in the race.
13. Iowa (R-Grassley) 
Although polling suggests that Tom Vilsack could make a race of things, it’s improbable that he’ll run unless Grassley retires — and what I’m hearing from Iowa constituents is that Grassley is unlikely to do so. (ed: Forgot that Vilsack will become Secretary of Agriculture. Still, if Grassley retires, there will be no shortage of strong Democratic challengers for the seat –nrs).
14. Delaware (D-Open)
15. Arizona (R-McCain)
16. New York (Jr.) (D-??) 
Long Island Congressman Peter King is apparently interested in challenging Caroline Kennedy — or whomever else Governor Paterson puts in this seat. Although King himself is not very likely to win, the fact that he’s willing to give up a House seat that he’ll probably never win back suggests that Republicans don’t consider the race off-limits. Democrats might not consider the seat off-limits either, potentially giving Kennedy a vigorous primary challenge.
17. Arkansas (D-Lincoln)
18. California (D-Boxer)
19. North Dakota (D-Dorgan)
20. Connecticut (D-Dodd)
21. Alaska (R-Murkowski) 
New polling in this race suggests that Lisa Murkowski is vulnerable, most certainly to a primary challenge by Sarah Palin and possibly in the general election too. But, the Democratic bench in Alaska is not strong.
22. Oklahoma (R-Coburn)
23. Wisconsin (D-Feingold)
24. Hawaii (D-Inoyue)
25. Georgia (R-Isakson)
26. Louisiana (R-Vitter)
27. Maryland (D-Mikulski)
28. South Carolina (R-DeMint)
29. Washington (D-Murray)
30. Vermont (D-Leahy)
31. Alabama (R-Shelby)
32. South Dakota (R-Thune)
33. Indiana (D-Bayh)
34. Oregon (D-Wyden)
35. Utah (R-Bennett)
36. New York (Sr.) (D-Schumer)
37. Idaho (R-Crapo)