By Patricia Zengerle

WASHINGTON, May 10 (Reuters) - U.S. Senate Democrats released a letter from Secretary of Defense Lloyd Austin on Wednesday saying a Republican's decision to hold military nominations harms national security, intensifying a dispute over the military's abortion policy.

Republican Senator Tommy Tuberville has been blocking more than 150 military nominations from moving forward for months because he believes the Pentagon is improperly using funding to cover travel costs for abortions of service members.

After the U.S. Supreme Court last year overturned the landmark 1973 Roe v. Wade ruling that recognized a constitutional right to abortion, the Pentagon said it would cover travel costs for service members seeking abortions and up to 21 days off.

The military argues that women service members cannot choose where they are stationed.

"I appreciate and share your deep concern over this hold, which is unprecedented in its size and scope," Austin wrote in a letter to Senator Elizabeth Warren, who chairs the Senate Armed Services Committee's subcommittee on personnel. "Delays in confirming our general and flag officers pose a clear risk to U.S. military readiness, especially at this critical time."

"This indefinite hold harms America's national security and hinders the Pentagon's normal operations," Austin wrote.

Senior military nominations are approved by the committee and eventually the Senate. Although the review is usually routine, a single senator can pause the process by putting a hold on nominations that force them to be considered one at a time, taking many hours each.

A spokesperson for Tuberville said the senator's position has not changed, citing his past comments in the Senate denying that his hold had any effect on readiness.

"Democrats keep repeating the same talking points — and the same opinion. But not one of them has cited any facts. Not one. I've even asked the Pentagon to explain to me how this affects readiness. All I've heard is opinions," Tuberville said.

Tuberville has argued that the Pentagon changed its policy without congressional approval. He said he would continue to hold the nominees until the Pentagon changes its policy or Congress changes the law. (Reporting by Patricia Zengerle; Editing by Richard Chang)