Outcome of Hagel filibuster vote uncertain

Washington (CNN) - Chuck Hagel’s bid to lead the Pentagon faces an uncertain outcome on a critical vote Friday as Democrats attempt to break a GOP filibuster of the defense secretary nominee.

Despite optimism from administration officials, who believe Hagel will get the five GOP votes he needs to end the filibuster, senior congressional sources in both parties are less positive. They told CNN they now are not sure he will get the votes.

Doubts were raised Wednesday after Sen. John McCain, R-Arizona, said he was reconsidering his pledge not to filibuster Hagel because the Obama administration was refusing to provide key details about the president’s actions on the night of the attack on the U.S. consulate in Benghazi.

McCain’s vote is important because he is one of the leading Republicans on military matters and other senators are expected to follow his lead. But he was also one of just five GOP senators who publically announced they would not filibuster Hagel. If he supports the filibuster, it could be impossible for Hagel to get 60 votes.

“I’ve always said I just want an answer to the question,” McCain explained. “We deserve answers before we move forward with nominees. That’s been the standard I’ve pursued for the past 26 years.”

Senate Armed Services Committee Chairman Carl Levin, D-Michigan, said he expects the administration will provide a response to the Benghazi question before Friday’s vote.

Filibusters of cabinet officials are extremely rare, largely because senators typically believe a president has a right to pick the leaders of his government.

“This is the first time in the history of our country that a presidential nominee for secretary of defense has been filibustered,” Senate Majority Leader Harry Reid said. “What a shame.”

The administration isn't budging, and an official insisted to CNN Hagel won't withdraw his nomination.