* At least three Republicans now plan to vote for Hagel
* White House says delays threaten national security
* Confirmation vote in Senate expected next week (Adds quote, Inhofe letter, details.)
By Patricia Zengerle
WASHINGTON, Feb 21 (Reuters) - Chuck Hagel's path to confirmation as President Barack Obama's new secretary of defense became more secure on Thursday when Republican Senator Richard Shelby said he would support the nomination.
Shelby joined almost every other Republican senator a week ago in delaying a vote on confirming Hagel in order to allow colleagues more time to examine Hagel's record, said spokesman Jonathan Graffeo.
He now will vote for a motion to stop debate, ending the delay, and in favor of the nomination, barring any surprises between now and a confirmation vote, Graffeo said.
Fifteen other Republican senators signed a letter to Obama on Thursday asking that he withdraw Hagel's nomination, saying they respected the military service of the decorated Vietnam War veteran but that he lacked the bipartisan support and confidence to serve effectively.
The White House said it still supported Hagel and expected he would be confirmed. The Senate is expected to vote next week.
Shelby, a five-term senator from Alabama, served with Hagel during the nominee's two terms as a Republican senator from Nebraska. He is at least the third Republican - along with Mike Johanns and Thad Cochran - to back Hagel's confirmation.
Shelby's support for Hagel was first reported by the Decatur Daily newspaper. "He's probably as good as we're going to get," he told the Alabama paper.
Democrats control 55 votes in the 100-member Senate, and none has come out against Hagel. While he has long looked likely to garner the 51 votes he needs, his backers feel it will strengthen him as Pentagon chief to have as much bipartisan support as possible. Hagel would succeed Leon Panetta as defense secretary.
Many Republicans have fiercely opposed Hagel's nomination as civilian chief at the Pentagon since it was announced on Jan. 7.
Hagel broke from his party as a senator by opposing former President George W. Bush's handling of the Iraq war, infuriating some Republicans. Some have also raised questions about whether he is sufficiently supportive of Israel or tough enough on Iran.
Republicans also worry Hagel will be too supportive of any effort by Obama to include cuts in Pentagon spending as a way to deal with yawning U.S. budget deficits.
Hagel's performance at his confirmation hearing before the Senate Armed Services Committee drew harsh criticism. Even some Democrats said he appeared at times unprepared or hesitant in the face of aggressive questioning.
The pressure continued with Thursday's letter from the 15 Republicans, which cited among other things statements by the former senator that they said "proclaimed the legitimacy of the current regime in Iran."
The White House blasted what it called continued political posturing by Republicans.
"We firmly believe that Senator Hagel will be confirmed, but the waste of time is of consequence," White House spokesman Jay Carney said at his daily news briefing.
"There are 66,000 men and women in uniform in Afghanistan and we need our new secretary of defense on the job to be part of the significant decisions that have to be made as we bring that war to a responsible end," he said.
Many of the 15 senators who signed the letter have been among Hagel's most vocal opponents. They included half of the 12 Republicans on the Senate armed services panel - James Inhofe, the top Republican on the committee, as well as Lindsey Graham, Roger Wicker, David Vitter, Ted Cruz and Mike Lee.
The other nine were Senators John Cornyn, Patrick Toomey, Marco Rubio, Daniel Coats, Ron Johnson, James Risch, John Barrasso, Tom Coburn and Tim Scott.
Republican Senator John McCain, a member of the committee and one of Hagel's most outspoken critics, was not among the signatories. He said on Sunday that he did not believe Hagel's nomination should be held up any further.
Inhofe, who has contended that Hagel is "cozy" with Iran, also sent a "Dear Colleague" letter to his fellow Republican senators on Thursday lobbying for them to back a procedural tactic that would continue to delay the Senate vote on Hagel's confirmation, the Hill newspaper reported. (Editing by Todd Eastham)