Republicans who got reelected by less than 20%.  There have been several diaries on this theme, including one that showed the 50 best pickup opportunities for the Democrats in 2008.  Actually, this list suggests there are more opportunties than that: a stunning 83 of the 202 Republicans won by 20% or less.

CNN provided the totals for the amount of money the Democratic candidates raised up to, in most cases, October 18. I assume the money raised was also spent in primaries where necessary. Number in bracket is the percent vote the Republican incumbent received in 2004.

1.Florida 13, Vern Buchanan (open seat) contested Christine Jennings $2,050,748
2.North Carolina 8, Robin Hayes (55.5) 50.1-49.9, 0.2% Larry Kissell $451,430
3.New Mexico 1, Heather Wilson (54.4%) 50.2-49.8, 0.4% Patricia Madrid $2,909,778
4.Ohio 15, Deborah Pryce (60.0) 50.2-49.7, 0.5% Mary Jo Kilroy $2,383,165
5.Wyoming, Barbara Cubin (55.3) 48.3-47.8, 0.5% Gary Trauner $798,590
6.Ohio 2, Jean Schmidt (51.6, special election) 50.5-49.4, 1.1% Victoria Wulsin $691,197
7.Pennsylvania 6, Jim Gerlach (51.0) 50.7-49.3%, 1.4% Lois Murphy $3,442,856
8.New Jersey 7, Mike Ferguson (57.0) 49.4-47.9, 1.5% Linda Stender $ 1,575,916
9.New York 25, Jim Walsh (90.4 no Dem in 2004) 50.8-49.2, 1.6%
Dan Maffei
10.Nevada 3, Jon Porter (54.4) 48.5-46.6, 1.9% Tessa Hafen $1,258,115
11.Colorado 4, Marilyn Musgrave (51.0) 45.6-43.1, 2.5% Angie Paccinone $1,544,947
12.Illinois 6, Peter Roskam (open seat) 51.4-48.6, 2.8%, Tammy Duckworth $3,269,173
13.Virginia 2, Thelma Drake (55.1) 51.3-48.5, 2.8% Phil Kellam $1,303,676
14.New York 29, Randy Kuhl (50.7) 51.5-48.5, 3.0% Eric Massa $ 1,140,213
15.Washington 8, Dave Reichert (51.5) 51.5-48.5, 3.0% Darcy Burner $2,413,070
16.California 4, John Doolittle (65.5) 49.1-45.9, 3.2% Charles Brown $1,171,328
17.Connecticut 4, Chris Shays (52.4) 51.0-47.6, 3.4% Diane Farrell $2,740,473
18.Michigan 7, Tim Walberg (open seat) 49.9-46.0, 3.9% Sharon Renier $39,394
19.New York 26, Tom Reynolds (55.6) 52.0-48.0, 4.0% Jack Davis $1,891,316
20.Ohio 1, Steve Chabot (59.8) 52.2-47.8, 4.4% John Cranley
21.Idaho 1, Bill Sali (open seat) 49.9-44.8, 5.1% Larry Grant $570,600
22.Michigan 9, Joe Knollenberg (58.5) 51.6-46.2 5.4% Nancy Skinner $330,139
23.Nevada 2, Dean Heller (open seat) 50.3-44.9, 5.4% Jill Derby  
24.Illinois 10, Mark Kirk (64.1) 53.4-46.6, 6.8%, Daniel Seals $1,429,541
25.Florida 8, Rik Keller (60.5) 52.8-45.7, 7.1% Charlie Stuart $917,543
26.Minnesota 6, Michelle Bachman (open seat) 50.1-42.1, 8.0% Patty Wetterling $2,720,145
27.Kentucky 4, Geoff Davis (54.4) 51.7-43.4, 8.1% Ken Lucas $1,289,584
28.Arizona 1, Rick Renzi (58.5) 51.8-43.4, 8.2% Ellen Simon $1,277,792
29.Indiana 3, Mark Souder (69.2) 54.3-45.7, 8.6% Thomas Hayhurst $639,086
30.Nebraska 2, Lee Terry (61.1) 54.7-45.3, 9.4% Jim Esch $384,974
31.California 50, Brian Bilbray (49.3, special election) 53.1-43.5, 9.6% Francine Busby
32.Nebraska 3, Adrian Smith (open seat) 55.0-45.0, 10.0% Scott Kleeb $719,894
33.Pennsylvania 15, Charlie Dent (58.6) 53.6-43.5, 10.1% Charles Dertinger
34.Illinois 11, Jerry Weller, (58.7) 55.1-44.9, 10.2%, John Pavich $532,323
35.Kentucky 2, Ron Lewis (67.9) 55.4-44.6, 10.8% John 'Mike' Weaver $762,532
36.New Jersey 5, Scott Garrett (57.6) 54.9-43.8, 11.1% Paul Aronsohn  
37.Michigan 11, Thad McCotter (57.0) 54.1-43.0, 11.1% Tony Trupiano $90,202
38.Pennsylvania 3, Phil English (60.1) 53.6-42.0, 11.6% Steve Porter $72,673
39.Florida 9, Gus Bilirakis (open seat) 55.9-44.1, 11.8% Phyllis Busansky $1,174,282
40.Virginia 11, Tom Davis (60.2) 55.5-43.6, 11.9% Andy Hurst $329,021
41.New York 3, Pete King (63.0) 56.0-44.0, 12.0% Dave Mejias $664,394
42.Michigan 8, Mike Rogers (61.1) 55.3-42.9, 12.4% Kim Marcinkowski $473,961
43.Florida 15, Dave Weldon (65.4) 56.3-43.7 12.6% Bob Bowman $91,279
44.Washington 5, Cathy McMorris (59.7) 56.4-43.6, 12.8% Peter Goldmark $901,862
45.New York 13, Vito Fossella (59.0) 56.8-43.2, 13.6% Steve Harrison $109,858
46.Ohio 5, Paul Gillmor (67.0) 56.8-43.2, 13.6% Robin Weirauch $83,258
47.Iowa 4, Tom Latham (60.9) 57.2-42.8, 14.4% Selden Spencer
48.North Carolina 5, Virginia Foxx (58.8%) 57.2-42.8, 14.4% Roger Sharpe $111,678
49.West Virginia 2, Shelley Moore Capito (57.5) 57.2-42.8, 14.4% Mike Callaghan $502,506
50.Ohio 12, Pat Tiberi (65.4) 57.3-42.7, 14.6% Bob Shamansky $1,639,454 (mostly self financed)
51.Texas 10, Mike McCaul (no Dem in 2004) 55.3-40.4, 14.9% Ted Ankrum $48,718
52.Texas 32, Pete Sessions (54.3) 56.4-41.3, 15.1% Will Pryor $462,088
53.Illinois 15,Tim Johnson (61.1)  57.6-42.4, 15.2% David Gill $209,090
54.Pennsylvania 18, Tim Murphy (62.8) 57.8-42.2, 15.6% Chad Kluko $95,320
55.Florida 24, Tom Feeney (unopposed in 2004) 57.9-42.1, 15.8% Clint Curtis $44,356  
56.Minnesota 2, John Kline (56.4) 56.2-40.0 16.2% Coleen Rowley $562,821
57.Virginia 10, Frank Wolf (63.8) 57.3-41.0, 16.3% Judy Feder $1,244,916
58.Illinois 13, Judy Biggert (65.1) 58.3-41.7, 16.6%, Joseph Shannon $55,719
59.Ohio 16, Ralph Regula (66.5) 58.3-41.7, 16.6%, Thomas Shaw
60.Alaska, Don Young (71.1) 56.6-40.0, 16.6% Diane Benson $104,209
61.Nebraska 1, Jeff Fortenberry (54.2) 58.4-41.6 16.8% Maxine Moul $894,146
62.Florida 25, Mario Diaz Balart (unopposed in 2004) 58.5-41.5, 17.0% Michael Calderin $34,940
63.Ohio 3, Mike Turner (62.3) 58.5-41.5, 17.0% Richard Cheema $349,196
64.Pennsylvania 16, Joseph Pitts (64.3) 56.6-39.5, 17.1% Lois Herr $266,232
65.New Jersey 3, Jim Saxton (63.4) 58.4-41.0 17.4% Rich Sexton $135,579
66.Delaware, Mike Castle (69.1) 57.2-38.8, 18.4% Dennis Spivack $364,149
67.Ohio 14, Steve LaTourette (62.7) 57.6-39.1, 18.5% Lewis Katz $197,741
68.Colorado 6, Tom Tancredo 58.6-39.9, 18.7% Bill Winter $694,536
69.New Mexico 2, Steve Pierce (60.2) 59.4-40.5, 18.9% Al Kissling $147,164
70.Florida 21, Lincoln Diaz Balart (72.8, no Democrat in 2004) 59.5-40.5, 19.0% Frank Gonzalez
71.California 26, David Drier (53.6) 57.0-37.9, 19.1% Cynthia Matthews $54,484
72.Kentucky 1, Ed Whitfield (67.4) 59.6-40.4, 19.2% Tom Barlow $30,015
73.Colorado 5, Doug Lamborn (open seat) 59.6-40.4, 19.2% Jay Fawcett $478,944
74.Virginia 5, Virgil Goode (63.7) 59.1-39.9, 19.2% Al Weed $545,216
75.Illinois 14, Denny Hastert (68.6) 59.8-40.2, 19.6% Jonathan Laesch $230,481
76.Arizona 2, Trent Franks (59.2) 58.6-38.9, 19.7% John Thrasher $32,352
77.Texas 31, John Carter (64.8) 58.5-38.8, 19.7% Mary Beth Harrell $168,122
78.Florida 5, Ginny Brown-Waite (65.9) 59.9-40.1, 19.8% John Russell $79,803
79.Florida 6, Cliff Stearns (64.4) 59.9-40.1, 19.8% David Bruderly $123,025
80.Washington 4, Doc Hastings (62.6) 59.9-40.1, 19.8% Richard Wright $242,3160
81.Montana, Denny Rehberg (64.4) 58.9-39.1, 19.8% Monica Lindeen $490,634
82.Ohio 4, Jim Jordan (open seat) 60.0-40.0, 20.0% Richard Siferd $132,164
83.Indiana 6, Mike Pence (67.0) 60.0-40.0, 20.0% Barry Welsh $49,414

Republican incumbents who actually increased their vote percentage. In most cases, there are explanations for this. ...................2006...2004
1.Texas 1, Louie Gohmert 68.0%  61.5% (defeated Democratic incumbent in 2004 after redistricting)
2.Texas 2, Ted Poe 55.5% 65.6% (defeated Democratic incumbent in 2004 after redistricting)
3.Texas 11, Mike Conaway unopposed, 76.8% (self explanatory :D)
4.Texas 19, Randy Neugebauer 67.7% 58.4% (defeated Democratic incumbent in 2004 after redistricting)
5.Texas 32, Pete Sessions 56.4% 54.3% (defeated Democratic incumbent in 2004 after redistricting)
6.Nebraska 1, Jeff Fortenberry 58.4% 54.2% (Freshman)
7.California 26, David Drier 57.0% 53.6%
8.California 42, Gary Miller unopposed, 68.2%
9.California 48, John Campbell 59.9% 44.7% (special election with strong 3rd party candidate)
10.California 49, Darrell Issa 63.3% 62.6%
11.California 50, Brian Bilbray 53.1% 49.3%
12.Arkansas 3, John Boozman 62.4% 59.3%
13.Tennessee 3, Zach Wamp 65.7% 64.7%
14.Alabama 1, Jo Bonner 68.1% 63.1% (freshman)
15.Virginia 4, Randy Forbes 76.1% 64.5% (no Democratic opponent :D)
16.Gergia 11, Phil Gingrey 71.1% 57.4% (freshman)
17.Florida 12, Adam Putnam 69.1% 64.9% (no Democratic opponent :D)
18.Oklahoma 1, John Sullivan 63.6% 60.2% (freshman)
19.Wisconsin 6, Tom Petri 98.9% 67.0% (no Democratic opponent)
20.New York 29, Randy Kuhl 51.5% 50.7% (3rd party candidate in 2004)
21.Minnesota 3, Jim Ramstad 64.9% 64.6%
22.Louisiana 1, Bobby Jindal 88.1% 78.4%
23.Louisiana 5, Rodney Alexander 68.3% 59.4% (party switcher in 2004)
24.Louisiana 6, Richard Baker 82.8% 72.2%  

Incumbent Democrats reelected by under 20%, percent in bracket is 2004 vote percentage
1.Illinois 8, Melissa Bean 50.9-44.0% (51.7)
2.Texas 17, Chet Edwards 58.1-40.3% (51.2%)
3.Texas 27, Soloman Ortiz 56.8-38.9% (63.1%)
4.Indiana 7, Julia Carson 53.8-46.2% (54.4%)
5.Oregon 5, Darlene Hooley 54.0-42.8% (52.9%)
6.Iowa 3, Len Boswell 51.9-46.5% (55.2%)
7.South Carolina 5, John Spratt 56.9-43.1% (63.0)
8.Georgia 8, Jim Marshall 50.5-49.5% (62.9%) redistricted
9.Georgia 12, John Barrow 50.3-49.7% (51.8%)
10.Louisiana 3, Charlie Melancon 55.0-40.3%

Incumbent Democrats whose share of vote declined from 2004.  There are a surprising number of them.  In about 1/4 of the cases there was no Republican opponent in 2004. First number is 2006 voting percentage, second number is 2004 percentage.

1.Illinois 1, Bobby Rush, 84.1 84.9
2.Illinois 2, Jesse Jackson Jr, 84.8 88.5 (no Republican in 2004)
3.Illinois 8, Melissa Bean, 50.9 51.7 (3rd party candidate in 2006)
4.Illinois 9, Jan Schakowsky, 74.6 75.7
5.Ohio 11, Stephanie Tubbs Jones 83.4 100.0 (no Republican in 2004)
6.Indiana 7, Julia Carson 53.8 54.4
7.Texas 18, Sheila Jackson Lee, 76.6 88.9 (no Republican in 2004)
8.Texas 25, Lloyd Doggett, 67.3 67.6
9.Texas 27, Soloman Ortiz, 56.8 63.1
10.Texas 29, Gene Green, 73.5 94.1 (no Republican in 2004)
11.Texas 30, Eddie Bernice Johnson, 80.2 93.0 (no Republican in 2004)
12.West Virginia 1, Alan Mollohan, 64.3 67.8
13.North Carolina 7, Mike McIntyre, 72.8 73.2
14.Pennsylvania 11, Paul Kanjorski 72.5 94.1 (no Republican in 2004)
15.Pennsylvania 12, John Murtha, 60.8 100.0 (no Republican in 2004 :D)
16.Pennsylvania 14, Mike Doyle 89.8 100.0 (no Republican in 2004 or 2006)
17.California 1, Mike Thompson, 66.2 67.0
18.California 6, Lynn Woolsey 70.2 72.7
19.California 8, Nancy Pelosi 80.4 84.7
20.California 18, Dennis Cardoza 65.5 67.5
21.California 29, Adam Schiff 63.5 64.7
22.California 32, Hilda Solis, 83.0 85.1 (no Republican in 2004 or 2006)
23.California 38, Grace Napolitano 75.3 100.0 (no Republican in 2004)
24.California 43, Joe Baca 64.5 66.4
25.Iowa 3, Len Boswell 51.9 55.2
26.Missouri 1, William 'Lacy' Clay 72.9 75.3
27.South Carolina 5, John Spratt 56.9 63.0
28.South Carolina 6, Jim Clyburn 64.4 67.0
29.Arkansas 4, Mike Ross 74.7 100.0 (no Republican in 2004)
30.Tennessee 5, Jim Cooper 69.0 69.3
31.Tennessee 8, John Tanner 73.2 74.3
32.Georgia 8, Jim Marshall 50.5 62.9 (redistricting in 2006)
33.Georgia 12, John Barrow, 50.3 51.8 (redistricting in 2006)
34.Georgia 13, David Scott, 69.2 100.0 (no Republican in 2004)
35.Nevada 1, Shelley Berkley 64.8 66.0
36.Massachusetts 1, John Olver, 76.4 99.0 (no Republican in 2004 or 2006)
37.Massachusetts 6, John Tierney 69.6 69.9
38.Massachusetts 8, Mike Capuano 90.7 98.7 (no Republican in 2004 or 2006)
39.Massachusetts 9, Stephen Lynch 78.1 99.0 (no Republican in 2004)
40.Massachusetts 10, Bill Delahunt 64.3 65.9
41.Arizona 7, Raul Grijalva 61.1 62.1
42.Rhode Island 2, Jim Langevin 72.7 74.5
43.Wisconsin 2, Tammy Baldwin 62.8 63.3
44.Wisconsin 7, David Obey 62.2 85.6 (no Republican in 2004)
45.Minnesota 8, Jim Oberstar 63.6 65.2  

New Democratic Representatives. Dollar figures are from CNN
A. Democratic  (and those who caucused with the Dems) open seats
1.Illinois 17, Phil Hare 57.2-42.8% $692,854
2.Ohio 6, Charlie Wilson 62.1-37.9% $1,742,264
3.Ohio 13, Betty Sutton 61.2-38.8% $1,196,015
4.Tennessee 9, Steve Cohen 59.9-18.0% $640,266
5.New Jersey 13, Albio Sires 77.5-19.4% $1,640,913
6.Georgia 4, Hank Johnson 75.3-24.6% $693,992
7.Florida 11, Kathy Castor 69.7-30.3% $1,285,607
8.Maryland 3, John Sarbanes 64.0-33.8% $1,319,754
9.New York 11, Yvette Clarke 90.0-7.6% $519,188
10.Vermont, Peter Welch 53.2-44.5% $1,862,494
11.Minnesota 5, Keith Ellison 55.6-21.3% $623,563
12.Hawaii 2, Mazie Hirono 61.0-39.0% $1,060,944

Republican open Seats
1.Texas 22, Nick Lampson 51.8-41.8% $3,387,122
2.Ohio 18, Zack Space 62.1-37.9% $1,234,487
3.Iowa 1, Bruce Braley 55.1-43.2% $1,944,548
4.Florida 16, Tim Mahoney 49.5-47.7% $1,780,983
5.Colorado 7, Ed Perlmutter 54.9-42.1% $2,404,037
6.Arizona 8, Gabrielle Giffords 54.3-42.1% $2,184,911
7.Wisconsin 8, Steve Kagen 50.9-48.8% $2,229,943
8.New York 24, Mike Arcuri 53.9-45.0% $1,660,480

Defeated Republican incumbents (the number in brackets is the percent of the vote the Republican incumbent received in 2004.
1.Kentucky 3, John Yarmuth, 50.6-48.2% (60.3) $1,682,278
2.Indiana 2, Joe Donnelly, 54.0-46.0% (54.2) $1,333,572
3.Indiana 8, Brad Ellsworth 61.0-39.0% (53.4) $1,577,181
4.Indiana 9, Baron Hill 50.0-45.7% (49.5) $1,672,524
5.North Carolina 11, Heath Shuler 53.8-46.2% (54.9) $1,632,945
6.Pennsylvania 4, Jason Altmire 51.9-48.0% (63.1) $686,272
7.Pennsylvania 7, Joe Sestak 56.4-43.6% (58.8) $2,706,636
8.Pennsylvania 8, Patrick Murphy 50.3-49.7% (55.3) $1,934,384
9.Pennsylvania 10, Chris Carney 52.9-47.0% (92.8) no Democrat in 2004) $1,164,155
10.Texas 23, Ciro Rodriguez won in runoff
11.California 11, Jerry McNerney 53.3-46.7% (61.3) $1,584,258
12.Iowa 2, Dave Loesback, 51.4-48.6% (58.9) $368,634
13.Florida 22, Ron Klein 50.9-47.1% (62.8) $3,595,978
14.Connecticut 2, Joe Courtney 50.0-50.0% (54.2) $2,012,685
15.Connecticut 5, Chris Murphy 56.5-43.5% (59.8) $2,196,278
16.Kansas 2, Nancy Boyda 50.6-47.1 (56.1) $528,132
17.Arizona 5, Harry Mitchell 50.4-46.4 (59.5) $1,447,591
18.New Hampshire 1 Carol Shea Porter 51.3-48.7 (63.3) $198,043
19.New Hampshire 2 Paul Hodes 52.8-45.6 (58.3) $1,246,059
20.New York 19, John Hall 51.2-48.8 (66.7) $1,061,455
21.New York 20, Kristin Gillibrand 53.1-46.9 (65.8) $2,043,090
22.Minnesota 1, Tim Walz, 52.7-47.1 (59.6) $869,980

What's most interesting is the seeming randomness of the Republican defeats. 8 of them won with over 60% of the vote in 2004 (I know a couple of them had corruption problems, one or two others may have faced very weak opponents in 2004 as well).