No Senator is vulnerable in his or her own right. I.e., can be defeated by a lesser-known candidate.
Likely resignations: Clinton (D-NY), Corzine (D-NJ), Hutchinson (R-TX), Frist (R-TN), Chafee (R-RI)
Hillary has a simple calculus. If Rudy runs, she loses, if Rudy doesn't run, she wins. If she loses, she will never be president. If she does not run, the Democrats will retain this seat unless Pataki or Rudy go for it.Corzine will resign to run for the Governorship and will very likely win. This is an outside shot since the NJ Dems are in such disarray and disrepute.
Kay Bailey will probably run for governor but her replacement is very likely to be R.
Frist is running for President. The Dems want this seat. Harold Ford wants it. Not bloody likely.
Chafee will switch to whatever party will elect him. For partisan purposes, this might as well be a resignation.
Generally vulnerable to a celebrity candidate:
Bingaman (D-NM), Nelson (D-FL), Dayton (D-MN), Conrad (D-ND)
These are the weakest of the pack and if a well-known celebrity candidate, say a former popular pol or a sports figure, were to run against them, they might lose.
Vulnerable to a specific candidate:
Kohl (D-WI) to T. Thompson, Jeffords (I-VT) to Howard Dean (no kidding), Clinton (D-NY) to Rudy, Nelson (D-FL) to Jeb (rumor is that Katherine Harris has been promised a clear shot at this seat which would be great, she would draw all Democrat money to Florida).
Death watch:
If Kennedy cannot run, Mitt Romney or Bill Weld could win the seat. If Byrd cannot run, that is a better than even R pickup.
For Mass., think Ambassador Ray Flynn...