His nomination, which Mr. Obama announced earlier this month, has been controversial from the get-go: Even before the decision was official, a handful of Republicans were threatening to pitch a fight over it, denouncing the former senator's positions on Israel and Iran and in some cases pledging immediately to vote against him. Outside advocacy groups were also swift to get involved, spending hundreds of thousands of dollars on television ads aimed at torpedoing Hagel's chances.
"I will not support Chuck Hagel's nomination," said Sen. John Cornyn, R-Tex., a member of the Armed Services Committee, in a statement released prior to the official announcement. "His record and past statements, particularly with respect to rogue nations like Iran, are extremely concerning to me."
Of particular concern to Republicans and some Democrats was Hagel's past opposition of some sanctions for Iran; for his having taken stances on Hezbollah and Hamas that critics have decried as overly lenient; and for criticism of what he called "the Jewish lobby," which invoked the ire of pro-Israel advocates. Additionally, Hagel came out as a vocal critic of former President George W. Bush's policies in Iraq.
Democrats had their own, added gripes with the pick: In addition to being a Republican, Hagel was targeted for making anti-gay comments about an ambassadorial nominee in 1998, whose nomination he opposed for being "openly, aggressively gay." He also voted on multiple occasions to limit abortion access for American servicewomen abroad.
In the days since his nomination, Hagel has rushed to assuage Democrats' concerns, meeting one-on-one with various senators in pursuit of their support, and signaling his commitment to fall in line with Mr. Obama's ideologies.
"I think from the personal conversations that senators have had with him, they expect that his testimony will show he will be committed to fully executing and implementing the administration's policies," said one Senate Democratic aide familiar with some of these conversations. "He's been well-prepared in his one-on-one meetings with the senators. I think he's been able to address the concerns that they've had, and he has given his very direct commitment on several issues."
Sen. Charles Schumer, D-N.Y., who had expressed concerns with Hagel's positions on Israel and Iran, as well as gay rights and abortion, was considered the most potent bastion of Democratic resistance to the nomination; his eventual support for Hagel was considered a signal that Democrats were ready to accept the choice. Since then, none have expressed plans to vote no on Mr. Obama's pick for defense secretary.
Hagel's efforts to win support from his own party's members, however, have been met with less success: Despite meetings with Sen. James Inhofe, R-Okla., the top Republican on the Armed Services Committee, as well as Sen. John McCain, R-Ariz., neither candidate has promised to support him. Inhofe stands firm in his opposition, and McCain said recently he still needs to "see what happens in the hearing" before making up his mind.
Today, a handful of Republicans on the Armed Services Committee sent Hagel a letter demanding answers on a number of questions related to his finances, and Sen. Lindsey Graham, R-S.C., threatened to put a hold on Hagel's nomination until Democrats agreed to hold a hearing with outgoing CIA Director Leon Panetta to discuss the U.S. response to the September attack on the U.S. consulate in Benghazi, Libya. Democrats subsequently agreed to hold this hearing, and Graham has withdrawn that threat, though he has not signaled that he will vote for Hagel.
At the moment, it's unclear how many Republicans, if any, will vote for Hagel: Donelle Harder, a spokeswoman for Inhofe, said today's hearings will probably be "the decision-maker" for a number of those who have yet to weigh in.
"There are a lot of unanswered questions," she told CBSNews.com. "I don't expect it to get bloody, but I do expect that you'll see some very smart, direct questions."
Still, Hagel only needs 14 out of 26 votes to clear the Armed Services Committee, which means that if all Democrats vote in his favor, he can squeak by without a single Republican vote.
Inhofe, at least, says he's ready to work with his fellow Republicans if, as many expect, he makes it through the process.
"When Hagel came to meet with the senator in person in his office, he let him know personally that he was going to be opposing him," Harder said. "But should Hagel get confirmed, he's prepared to have a working relationship with him."