REUTERS/Kevin Lamarque Former Sen. Chuck Hagel (R-NE) (L), sits down before giving testimony to the Senate Armed Services Committee to be Defense Secretary, on Capitol Hill in Washington, January 31, 2013. Hagel, 66, is a decorated Vietnam War veteran and a former two-term Republican senator. REUTERS/Kevin Lamarque (UNITED STATES - Tags: POLITICS MILITARY) 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.).

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.

Might President Obama heed the call and withdraw his nomination of Hagel?

Don't count on it, White House Press Secretary Jay Carney said Thursday.

“Absolutely not," Carney told reporters at Thursday's daily briefing when asked whether there's any chance the Hagel nomination will be withdrawn. "Any suggestion to the contrary might have been found in the minutes of the meetings of the Friends of Hamas.”

The "Friends of Hamas" mention was a dig at conservatives who have alleged that Hagel has ties to a group with that name. No such organization is believed to exist, and a New York Daily News reporter wrote Wednesday that he believes he may have inadvertently started the rumor by joking to a congressional source about the existence of such a group.

Carney went on to say that Hagel has the support of several leading Republicans, and added: "Unfortunately, however, some Senate Republicans put political posturing above our national security. For the first time in history, Republicans have filibustered a nominee to lead the Defense Department, a member of their own party. This runs against the majority will of the Senate and against our national interest. This waste of time is just meaningless political posturing. We believe Senator Hagel will be confirmed."

Here's the full letter:

February 21, 2013

President Barack H. Obama

The White House
1600 Pennsylvania Avenue NW
Washington, DC  20500

Dear President Obama:

Last Thursday, the Senate voted to continue its consideration of your nomination of former Senator Chuck Hagel to serve as our nation's next Secretary of Defense.  While we respect Senator Hagel's honorable military service, in the interest of national security, we respectfully request that you withdraw his nomination.

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.  Over the last half-century, no Secretary of Defense has been confirmed and taken office with more than three Senators voting against him.  Further, in the history of this position, none has ever been confirmed with more than 11 opposing votes.  The occupant of this critical office should be someone whose candidacy is neither controversial nor divisive.

In contrast, in 2011, you nominated Leon Panetta, who was confirmed by the Senate with unanimous support.  His Pentagon tenure has been a huge success, due in part to the high degree of trust and confidence that Senators on both sides of the aisle have placed in him.  The next Secretary of Defense should have a similar level of broad-based bipartisan support and confidence in order to succeed at a time when the Department of Defense faces monumental challenges, including Iran’s relentless drive to obtain nuclear weapons, a heightened threat of nuclear attack from North Korea, potentially deep budget cuts, a strategic pivot to the Asia-Pacific region, military operations in Afghanistan, the ongoing Global War on Terror, the continued slaughter of Syrian civilians at the hands of their own government, and other aftermath of the Arab Spring.

Likewise, Senator Hagel’s performance at his confirmation hearing was deeply concerning, leading to serious doubts about his basic competence to meet the substantial demands of the office.  While Senator Hagel's erratic record and myriad conversions on key national security issues are troubling enough, his statements regarding Iran were disconcerting.  More than once during the hearing, he proclaimed the legitimacy of the current regime in Tehran, which has violently repressed its own citizens, rigged recent elections, provided material support for terrorism, and denied the Holocaust.

Regarding U.S. policy on Iran's pursuit of nuclear weapons, Senator Hagel displayed a seeming ambivalence about whether containment or prevention is the best approach, which gives us great concern.  Any sound strategy on Iran must be underpinned by the highly credible threat of U.S. military force, and there is broad bipartisan agreement on that point.  If Senator Hagel becomes Secretary of Defense, the military option will have near zero credibility.  This sends a dangerous message to the regime in Tehran, as it seeks to obtain the means necessary to harm both the United States and Israel.

We have concluded that Senator Hagel is not the right candidate to hold the office of Secretary of Defense, and we respectfully request that you withdraw his nomination.  Thank you for your consideration.

Correction: An earlier version of this post said 14 GOP senators have signed the letter. It is actually 15. The post has been updated to reflect this.

David Nakamura and Felicia Sonmez contributed to this post.