| STATE | INCUMBENT |
| Alabama | Jeff Sessions (R) |
| Alaska | Ted Stephens (R) |
| Arkansas | Mark Pryor (D) |
| Colorado | Wayne Allard (R) is retiring |
| Delaware | Joe Biden (D) |
| Georgia | Saxby Chambliss (R) |
| Idaho | Larry Craig (R) says he won't seek re-election |
| Illinois | Dick Durbin (D) |
| Iowa | Tom Harkin (D) |
| Kansas | Pat Roberts (R) |
| Kentucky | Mitch McConnell (R) |
| Louisiana | Mary Landrieu (D) |
| Maine | Susan Collins (R) |
| Massachusetts | John Kerry (D) |
| Michigan | Carl Levin (D) |
| Minnesota | Norm Coleman (R) |
| Mississippi | Thad Cochran (R) |
| Montana | Max Baucus (D) |
| Nebraska | Chuck Hagel (R) is retiring |
| New Hampshire | John Sununu (R) |
| New Jersey | Frank Lautenberg (D) |
| New Mexico | Pete Domenici (R) is retiring |
| North Carolina | Elizabeth Dole (R) |
| Oklahoma | James Inhofe (R) |
| Oregon | Gordon Smith (R) |
| Rhode Island | Jack Reed (D) |
| South Carolina | Lindsey Graham (R) |
| South Dakota | Tim Johnson (D) |
| Tennessee | Lamar Alexander (R) |
| Texas | John Cornyn (R) |
| Virginia | John Warner (R) is retiring |
| West Virginia | Jay Rockefeller (D) |
| Wyoming | Michael Enzi (R) |
| Wyoming | John Barrasso (R) (filling the term of Sen. Craig Thomas, who died on June 4, 2007) |