By in theory, I meant regardless of whether they actually would run or could get nominated.
I've made some minor changes to show my opinions.
4.1 Retiring Democratic incumbents (3 seats)
4.1.1 Jeff Bingaman of New Mexico
D: Rep. Martin Heinrich
R:
Rep. Steven Pearce Fmr. Gov. Gary Johnson
4.1.2 Kent Conrad of North Dakota
D: ex.-Sen. Byron Dorgan
R: Atty. General Wayne Stenehjem
4.1.3 Jim Webb of Virginia
D:
ex.-Gov. Tim Kaine Fmr. Rep. Tom Perriello
R:
Rep. Frank Wolf Fmr. Rep. Tom Davis
4.2 Retiring Independent incumbent (1 seat)
4.2.1 Joe Lieberman of Connecticut
D: Rep. Chris Murphy
or Ned LamontR: ex-Gov. Jodi Rell
4.3 Retiring Republican incumbents (2 seats)
4.3.1 Jon Kyl of Arizona
D: Rep. Gabrielle Giffords if she makes a full recovery by then. Otherwise ex.-Gov. Janet Napolitano
R:
Gov. Jan Brewer Rep. Jeff Flake
4.3.2 Kay Bailey Hutchison of Texas
D: ex.-Houston mayor Bill White (won't matter)
R: pretty much any big name
Lt. Gov. David Dewhurst4.4 Democratic incumbents (18 seats)
4.4.1 Dianne Feinstein of California -
Rep. Mary Bono Mack Fmr. Rep. Tom Campbell
4.4.2 Tom Carper of Delaware - ex.-Rep. Mike Castle
4.4.3 Bill Nelson of Florida - ex.-Gov. Jeb Bush
4.4.4 Daniel Akaka of Hawaii - ex.-Gov. Linda Lingle (won't matter)
4.4.5 Ben Cardin of Maryland ex.-Gov. Bob Ehrlich (won't matter)
4.4.6 Debbie Stabenow of Michigan - Rep. Candice Miller
Or maybe Rep. Thad McCotter4.4.7 Amy Klobuchar of Minnesota - ex.-Gov. Tim Pawlenty
4.4.8 Claire McCaskill of Missouri - Rep. Jo Ann Emerson
4.4.9 Jon Tester of Montana - Rep. Denny Rehberg
4.4.10 Ben Nelson of Nebraska - Gov. Dave Heineman (though all 3 Reps would beat Nelson)
4.4.11 Bob Menendez of New Jersey - Gov. Chris Christie
Or Fmr. Gov. Christine Todd Whitman4.4.12 Kirsten Gillibrand of New York - ex.-NYC mayor Rudy Giuliani
Or Fmr. Gov. George Pataki4.4.13 Sherrod Brown of Ohio -
Rep. Pat Tiberi Lt. Gov. Mary Taylor
4.4.14 Bob Casey, Jr. of Pennsylvania - Rep. Jim Gerlach
Or Fmr. Gov. Tom Ridge4.4.15 Sheldon Whitehouse of Rhode Island - ex.-Gov. Don Carcieri (won't matter)
4.4.16 Maria Cantwell of Washington -
candidate Dino Rossi Rep. Dave Reichert
4.4.17 Joe Manchin of West Virginia - Rep. Shelley Moore Capito
4.4.18 Herb Kohl of Wisconsin - Rep. Paul Ryan
Or Fmr. Gov. Tommy Thompson4.5 Independent incumbent (1 seat)
4.5.1 Bernie Sanders of Vermont - ex.-Gov. Jim Douglas (won't matter)
4.6 Republican incumbents (8 seats)
4.6.1 Richard Lugar of Indiana - Rep. Joe Donnelly
4.6.2 Olympia Snowe of Maine - Rep. Mike Michaud (wouldn't matter against Snowe)
4.6.3 Scott Brown of Massachusetts - Rep. Jim McGovern
4.6.4 Roger Wicker of Mississippi - ex.-Gov. Ronnie Musgrave (won't matter)
4.6.5 John Ensign of Nevada - Rep. Shelley Berkley
4.6.6 Bob Corker of Tennessee - ex.-Gov. Phil Bredesen
4.6.7 Orrin Hatch of Utah - Rep. Jim Matheson (won't matter)
4.6.8 John Barrasso of Wyoming - ex.-Gov. Dave Freudenthal (won't matter)