WASHINGTON (AP) — Chuck Hagel has lined up the necessary votes for the Senate to confirm him next week to be the nation's next defense secretary, after a senior Republican lawmaker said he will back President Barack Obama's choice.

Barring any new developments, five-term Sen. Richard Shelby of Alabama said he would vote for his fellow Republican and the former two-term Nebraska senator, with the expectation that Hagel will win Senate approval. If confirmed, Hagel would succeed Defense Secretary Leon Panetta, who is stepping down after four years first as CIA director and then Pentagon chief.

"He's probably as good as we're going to get," Shelby told the Decatur Daily about Hagel.

Jonathan Graffeo, a spokesman for the senator, said Thursday that unless any new, damaging information to the nominee emerges between now and an expected Senate vote on Tuesday, Hagel has Shelby's vote.

Obama's choice has faced strong Republican opposition, and last week the GOP succeeded in an unprecedented filibuster of a nominee for defense secretary. In fresh evidence of the resistance, 15 Republican senators sent a letter to Obama on Thursday calling on him to withdraw the nomination.

"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. The letter came shortly after news of Shelby's support for Hagel.

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

White House spokesman Jay Carney on Thursday 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.

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"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.

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.

A 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 recess. But the administration is more confident about Hagel's prospects after private conversations with several senators to ensure Hagel has 60 votes, according to an official close to the confirmation process.

Several senators who voted to delay a vote last week — Shelby among them — are expected to allow the nomination to move forward next week.

Republicans have criticized Hagel for his past statements and votes, contending that he hasn't been sufficiently supportive of Israel and has been too tolerant of Iran. They also have challenged his support for reducing the nation's nuclear arsenal and his opposition to the Iraq war after his initial vote for the conflict.

His 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 Hagel responded to a separate Graham letter on Israel last week.

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Associated Press writer Darlene Superville contributed to this report.

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Follow Donna Cassata on Twitter: http://twitter.com/DonnaCassataAP