Former senator Chuck Hagel arrives to give testimony before the Senate Armed Services Committee on Jan. 31 on Capitol Hill. (Kevin Lamarque/Reuters )

Five days before Chuck Hagel is expected to be confirmed as Secretary of Defense, 15 GOP senators have signed a letter asking President Obama to withdraw his nomination.

"While we respect Sen. Hagel's honorable military service, in the interest of national security, we respectfully request that you withdraw his nomination," the senators wrote. "It would be unprecedented for a Secretary of Defense to take office without the broad base of bipartisan support and confidence needed to serve effectively in this critical position."

The letter is signed by Sens. John Cornyn (R-Tex.), Jim Inhofe (R-Okla.), Lindsey Graham (R-S.C.), Roger Wicker (R-Miss.), David Vitter (R-La.), Ted Cruz (R-Tex.), Mike Lee (R-Utah), Pat Toomey (R-Pa.), Marco Rubio (R-Fla.), Dan Coats (R-Ind.), Ron Johnson (R-Wis.), Jim Risch (R-Idaho), John Barrasso (R-Wyo.), Tom Coburn (R-Okla.) and Tim Scott (R-S.C.).