There are so few, I decided to include every race won by less than 20%.
I broke it up into 3 categories: long time incumbents, freshmen, and open seat winners.
The good news for Democrats is that there are more Republican held close districts than Democratic districts. That said, hardly any incumbents lose.
The order is by percent victory, from lowest to highest, not by percent of vote received by the winner.
The first number is the Percent win
The final number is the percent of vote the congressperson received in 2002
Republicans
Long term incumbents
1.Connecticut 4, Chris Shays, 4.8%, 52.4-47.6%, 64.4%
2.Minnesota 6,Mark Kennedy, 8.0% 54.0-46.0%, 57.3%
3.Connecticut 2, Rob Simmons,8.4%, 54.2-45.8%,54.1%
4.Indiana 8, John Hostettler, 8.9%, 53.4-44.5%, 51.3%
4.New Mexico 1, Heather Wilson, 8.9%, 54.4-45.5% 55.3%
6.North Carolina 11, Charles Taylor, 9.8%, 54.9-45.1% 55.5%
7.California 26, David Drier, 10.8%, 53.6-42.8%, 63.8%
8.North Carolina 8, Robin Hayes, 11.0%, 55.5-44.5%, 53.6%
9.New York 26, Tom Reynolds, 11.2%, 55.6-44.4%, 73.7%
10.Illinois 6, Henry Hyde, 11.6%, 55.8-44.2% 65.1%
11.Iowa 1, Jim Nussle, 11.9%, 55.2-43.3% 57.2%
12.Wyoming, Barbara Cubin, 13.5%, 55.3-41.8%, 60.5%
13.Texas 22, Tom Delay, 14.1%, 55.2-41.1% 63.2%
14.Kansas 2, Jim Ryun, 14.9%, 56.1-41.2%, 60.4%
15.New Jersey 7, Mike Ferguson, 15.3%, 57.0-41.7% 58.0%
16.West Virginia 2, Shelley Moore Capito, 16.2%, 57.5-41.3% 60.0%
17.Ohio 4, Mike Oxley, 16.8%, 58.6-41.4%, 67.5%
18.Illinois 11, Jerry Weller, 17.4%, 58.7-41.3% 64.3%
19.New York 13, Vito Fossella, 18.0%, 59.0-41.0%, 69.5%
20.Pennsylvania 7, Curt Weldon, 18.4%, 58.8-40.4% 66.1%
21.Michigan 9, Joe Knollenberg, 19.0%, 58.5-39.5%, 58.1%
22.Ohio 1, Steve Chabot, 19.7% 59.8-40.1%, 64.8%
23.Iowa 2, Jim Leach, 19.7% 58.9-39.2% 52.2%
Freshmen
1.Pennsylvania 6, Jim Gerlach, 2.0%, 51.0-49.0%, 51.4%
2.Colorado 4, Marilyn Musgrave, 6.2%, 51.0-44.8%, 54.9%
3.Indiana 2, Chris Chocola, 9.7%, 54.2-44.5%, 50.5%
4.Florida 13, Katherine Harris, 10.6%, 55.3-44.7%, 54.8%
5.Colorado 7, Bob Beauprez, 11.9%, 54.7-42.8%, 47.3%
6.Nevada 3, Jon Porter, 14.0%, 54.4-40.4%, 56.1%
7.Georgia 11, Phil Gingrey, 14.8%, 57.4-42.6%, 51.6%
8.Michigan 11, Thad McCotter, 16.0%, 57.0-41.0%, 57.2%
9.Minnesota 2, John Kline, 16.1%, 56.4-40.3%, 57.3%
10.New Jersey 5, Scott Garrett, 16.5%, 57.6-41.1%, 59.5%
Open Seats and defeated incumbent Democrats
1.Indiana 9, Mike Sodrel, 0.5%. 49.5-49.0%, Defeated Democratic incumbent
2.Washington 8, Dave Reichert, 4.8%, 51.5-46.7%
3.New York 29, Randy Kuhl, 9.9%, 50.7-40.8%
4.Louisiana 7, Charles Boustany, 10.0%, 55.0-45.0% (runoff result)
5.Texas 32, Pete Sessions, 10.3%, 54.3-44.0%, (incumbent vs. incumbent race)
6.Virginia 2, Thelma Drake, 10.3%, 55.1-44.8%
7.Kentucky 4, Geoff Davis, 10.5%, 54.4-43.9%
8.Nebraska 1, Jeff Fortenberry, 11.2%, 54.2-43.0%
9.Pennsylvania 8, Mike Fitzpatrick, 12.0%, 55.3-44.3%
10.Texas 2, Ted Poe, 12.6%, 55.5-42.9% (Defeated Democratic incumbent)
11.North Carolina 5, Virginia Foxx, 17.6% 58.8-41.2%
12.Texas 19, Randy Neugebauer, 18.3%, 58.4-40.1%
(incumbent vs. incumbent race)
13.Pennsylvania 15, Charlie Dent, 19.2%, 58.6-39.4%
14.Washington 5, Cathy McMorris, 19.4%, 59.7-40.3%
Democrats
Long Term Democrats
1.Texas 17, Chet Edwards, 3.8%, 51.2-47.4%, 51.6%
2.Oregon 5, Darlene Hooley, 8.6%, 52.9-44.3%, 54.7%
3.Iowa 3, Len Boswell, 10.5%, 55.2-44.7%, 53.4%
4.Indiana 7, Julia Carson, 10.7%, 54.4-43.7%, 53.1%
5.Kansas 3, Dennis Moore, 11.5%, 54.8-43.3%, 50.2%
6.Utah 2, Jim Matheson, 11.6%, 54.8-43.2%, 49.4%
7.Wisconsin 3, Ron Kind, 12.9%, 56.4-43.5%, 62.8%
8.Arkansas 2, Vic Snyder, 16.4%, 58.2-41.8%, 92.9% (no Republican opponent)
9.Mississippi 2, Bennie Thompson, 16.6%, 57.8-41.2%, 55.1%
10.Texas 15, Ruben Hinosjosa, 17.0%, 57.8-40.8%, uncontested
11.North Dakota, Earl Pomeroy, 19.2%, 59.6-40.4%, 52.4%
12.Oregon 1, David Wu, 19.4%, 57.5-38.1%, 62.7%
12.Maine 1, Tom Allen, 19.4%, 59.7-40.3% 63.8%
14.New Jersey 12, Rush Holt, 19.5%, 59.2-39.7% 61.0%
Freshmen
1.South Dakota, Stephanie Herseth, 7.5%, 53.4-45.9%, 50.6% (in special election)
2.Tennessee 4, Lincoln Davis, 11.1%, 54.6-43.5%, 52.1%
3.New York 1, Tim Bishop, 12.4%, 56.2-43.8%, 50.1%
4.North Carolina 13, Brad Miller, 17.6%, 58.8-41.2%, 54.7%
5.Maine 2, Michael Michaud, 18.5%, 58.0-39.5%, 52.0%
6.Kentucky 6, Ben Chandler, 18.6%, 58.6-40.0%, 55.1% (in special election)
Open Seats
1.Louisiana 3, Charlie Melancon, 0.4%, 50.2-49.8% (runoff results)
2.New York 27, Brian Higgins, 1.4%, 50.7-49.3%
3.Illinois 8, Melissa Bean, 3.4% 51.7-48.3% (defeated Republican incumbent)
4.Georgia 12, John Barrow, 3.6%, (defeated Republican incumbent)
5.Colorado 3, John Salazar, 4.0%, 50.6-46.6%
6.California 20, Jim Costa, 7.0%, 53.5-46.5%
7.Missouri 3, Russ Carnahan, 7.8%, 52.9-45.1%
8.Missouri 5, Emmanuel Cleaver, 13.1%, 55.2-42.1%
9.Pennsylvania 13, Allyson Schwartz, 14.4% 55.7-41.3%