Hagel cleared the threshold when five-term Republican Sen. Richard Shelby of Alabama said he would vote for the former GOP senator from Nebraska after joining other Republicans last week in an unprecedented filibuster of the Pentagon nominee.

“He’s probably as good as we’re going to get,” Shelby told the Decatur (Ala.) Daily.

Although a Republican, Hagel has faced strong GOP opposition, with many of his former colleagues voting last week to stall the nomination. Republicans have questioned Hagel’s support for Israel, tolerance of Iran and willingness to cut the nuclear arsenal. His opposition to the Iraq war after his initial vote for the conflict angered his onetime friend, Sen. John McCain, R-Ariz.

GOP lawmakers demanded more time to review the nomination that a divided Armed Services Committee had approved on a party-line vote.

Shelby’s support was a clear sign of weakening Republican opposition, and it prompted two letters within hours from Hagel’s fiercest GOP foes. One letter went to the president calling on him to withdraw the nomination, the other to GOP senators pleading with them to stand together against Hagel.

Fifteen Republicans senators wrote that Hagel lacks the bipartisan support and confidence to serve in the vital job of defense secretary.

“The occupant of this critical office should be someone whose candidacy is neither controversial nor divisive,” wrote the senators — all opponents of Hagel. Leading the effort was Texas Sen. John Cornyn, the party’s No. 2, who is up for re-election next year.

One name missing from the letter was McCain, who has called Hagel unqualified but indicated last Sunday that he wouldn’t stand in the way of a Senate vote.

Separately, the top Republican on the Armed Services Committee, Sen. Jim Inhofe of Oklahoma, sent a letter to his GOP colleagues urging them to vote again to block the nomination when the Senate returns from its recess next week. He acknowledged the reality that if the GOP fails to block a vote, Hagel proponents have the votes to approve him on an up-or-down vote.

“Make no mistake: A vote for cloture is a vote to confirm Sen. Hagel as secretary of defense,” wrote Inhofe. He said that while the Senate traditionally defers to presidents on their Cabinet choices, “our nation is at war. The Senate must insist on confirming only the most effective leaders.”

The Senate Republicans’ closed-door weekly meeting on Tuesday will be crucial to Inhofe’s hopes of keeping the GOP in line on Hagel.

At the White House, spokesman Jay Carney rejected GOP calls for Hagel to withdraw. He complained that Republicans were putting politics ahead of national security, pointing out that the administration wants Hagel to be part of decisions on the size of the U.S. force in Afghanistan as American and coalition forces wind down combat operations.

“This waste of time is not just meaningless political posturing because we firmly believe that Sen. Hagel will be confirmed. The waste of time is of consequence,” Carney told reporters.

The Senate also is holding up the nomination of John Brennan to be CIA director, with Republicans and Democrats seeking more information about the U.S. policy on the use of drones. Hagel and Brennan would join Secretary of State John Kerry in Obama’s overhauled, second-term national security team.

If confirmed, Hagel, a twice-wounded Vietnam combat veteran, would succeed Defense Secretary Leon Panetta, who is stepping down after four years first as CIA director and then Pentagon chief.

In a boost for Hagel’s nomination, former Republican leader Bob Dole, a decorated World War II veteran, issued a statement Thursday saying, “Hagel’s wisdom and courage make him uniquely qualified to be secretary of defense and lead the men and women of our armed forces. Chuck Hagel will be an exceptional leader at an important time.”

Hagel is expected to get all 55 Democratic votes and the support of three Republicans — Sens. Thad Cochran of Mississippi, Mike Johanns of Nebraska and Shelby. Two other Republicans — Sens. Susan Collins of Maine and Lisa Murkowski of Alaska — voted last week to allow the nomination to move ahead and are expected to do the same next week, giving Hagel the requisite 60 votes out of 100 necessary to end a filibuster.

An up-or-down vote on confirmation, with only a majority necessary, could occur as early as Wednesday.

The filibuster left the administration angry and troubled by the prospect of a nomination in limbo, with opposition groups redoubling their efforts to scuttle Hagel and the uncertainty of a weeklong Senate break. But the administration is more confident about Hagel’s prospects after private conversations with several senators to ensure Hagel gets past the 60-vote barrier, according to an official close to the confirmation process. The official spoke on condition of anonymity because he was not authorized to discuss private conversations.

Hagel’s nomination also has become entangled in GOP demands for more information from the Obama administration about the deadly assault on the U.S. diplomatic mission in Benghazi, Libya, last September that killed Ambassador Chris Stevens and three other Americans.

Sen. Lindsey Graham, R-S.C., has described Hagel as “radical” and pressed for Obama to abandon the nomination. Graham sent a new letter to Hagel this week with fresh questions about Israel, after Hagelresponded to a separate Graham letter on Israel last week.