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.).

Notably absent from the signatories is Sen. John McCain (R-Ariz.), a onetime close friend of Hagel's who has been among his chief critics in recent weeks.

Despite saying they will give Hagel an up-or-down vote Tuesday, Republicans and outside groups have kept up the pressure, hoping to reveal something that will disqualify Hagel. In recent days, there have been second-hand reports about Hagel's comments on Israel during speeches as Rutgers University, and a reporter for the conservative Weekly Standard tried unsuccessfully to get access to Hagel's archives in Omaha, Neb.

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