* At least three Republicans now plan to vote for Hagel
* White House says delays threaten national security
* Confirmation vote in Senate expected next week
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.
Fifteen other Republican senators signed a letter to Obama on Thursday asking that he withdraw Hagel's nomination, saying they respect the military service of the decorated Vietnam War veteran, but he lacks the bipartisan support and confidence to serve effectively.
The White House said it still supports Hagel and expects he will be confirmed. Senate Democrats expect a vote on his confirmation next week, after Congress returns from a recess, and that Hagel will win the majority support he needs to become the chief civilian at the Pentagon.
Graffeo said Shelby now plans to vote for a motion to stop debate, ending the delay, and in favor of the nomination when the Senate considers whether to confirm Hagel, barring any surprises between now and the vote.
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 say he will vote for Hagel.
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 to be confirmed, his backers feel it will strengthen him as Pentagon chief to have as much bipartisan support as possible.
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 also drew harsh criticism. Even some Democrats said he appeared at times unprepared or hesitant in the face of aggressive questioning.
The pressure against Hagel's nomination continued with Thursday's letter from the 15 Republicans, which cited among other things statements by the former senator they said "proclaimed the legitimacy of the current regime in Iran."
But the White House, blasting what it called continued political posturing by Republicans it contends puts the country at more risk, said there were no plans for Hagel's withdrawal.
"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 senators who signed the letter have been among Hagel's most vocal opponents. They included James Inhofe, the top Republican on the Senate armed services panel, and five other Republican members of that committee - 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.