If Not Hagel, Who?

Observers pitch Flournoy, Carter as alternatives if Hagel nomination fizzles.

Michele Flournoy, left, and Ashton Carter are likely on the shortlist if Chuck Hagel's nomination implodes.

Michele Flournoy, left, and Ashton Carter are old hands at the Defense Department, unlike Chuck Hagel.

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With just hours before the Senate Armed Services Committee is scheduled to vote on President Barack Obama's nominee for secretary of defense, a handful of Republicans are still threatening to walk out of the vote or filibuster the nomination of former Nebraska Republican Sen. Chuck Hagel.

"I would threaten to cause a 60-vote margin. Yes I would if it took a filibuster, I'd do it that way," Oklahoma Republican Sen. Jim Inhofe said during an appearance on Fox News Sunday. "I know he is popular, and I know that he is going to do his best. I am going to do my best to see that he is not going to be secretary of defense."

But if the Obama administration can't get Hagel, then who would they choose?

"I am convinced that Chuck Hagel is going to be the next Secretary of State," says John Nagl, a fellow Nebraskan and senior fellow at the Center for a new American Security.

[READ: Budget Cuts, Strategic Shift for Next Secretary]

But Nagl—who is credited with helping develop the counterinsurgency strategy implemented in Iraq and Afghanistan—says the Democrats' bench is stacked with strong contenders who are well respected and up to the task.

Michele Flournoy is on many experts' lists. The former under secretary of defense for policy was on the White House's short list in January, and she's broken down barriers amidst the male-dominated culture at the Department of Defense.

"She was working on policy on Afghanistan, Iraq, Pakistan and China. On the toughest problems the Pentagon faced, she was absolutely stoic and determined. She has enormous grace under pressure," says Nagl, who with Flournoy founded CNAS during Obama's presidential run. "You can talk to anyone in uniform or out of it who will tell you the same thing."

The other name on the docket is Ashton Carter, the deputy secretary of defense, who defense analysts say is one of the brightest and most tech savvy in the department. A former Rhodes Scholar with a PhD in Physics, Carter has co-authored or edited nearly a dozen books on military technology.

[RELATED: Hagel Underwhelms at Confirmation Hearing]

"Carter probably understands the administration's defense policies and postures better than anybody else," says Loren Thompson, a defense analyst for the Lexington Institute. "He has been the chief operating officer in Panetta's Pentagon. He has run things while Panetta's done outreach."

Thompson says Carter is more than qualified for the post, willing to serve, and is an expert on technology. The White House, however, was looking for someone with a bipartisan appeal, someone to recreate the defense strategy of President Bill Clinton's administration which included a former Republican Maine Sen. William Cohen as secretary of defense, a man credited with bridging the gap between a Democratic White House and Republican-held Congress.

But unlike Cohen's nomination, which passed unanimously, Republicans have not been happy with Hagel even with the "R" in front of his name. Many Republicans have already vowed that they will vote against Hagel and some have threatened to filibuster the secretary of defense nominee—an unprecedented move.

[OPINION: Professional Gadfly Hagel Was a Bizarre Choice]

Chairman of the Senate Armed Services Committee Carl Levin of Michigan had postponed a vote for Hagel last week, but announced Monday a vote will be held Tuesday to move Hagel's nomination to the Senate floor. Republicans wanted to see more financial records before the floor vote and South Carolina Republican Sen. Lindsey Graham demanded more answers on the death of U.S. ambassador to Libya Christopher Stephens and his security contingent in Benghazi. But while some Republicans have threatened to walk out on the vote, they're outnumbered by Democrats who can advance the nomination without a single GOP vote in committee.

So far, it appears that the Democrats are voting "yes" for Hagel's nomination as well as the Senate's two independent Senators. That brings the number to 57. Democrats will need 60 votes to break the filibuster.