U.S. President Joe Biden's nominee to head the Federal Aviation Administration (FAA) has withdrawn his nomination, in a setback for the administration that comes after Republican criticism that he was not qualified to serve as the top aviation regulator.

Last year, Biden nominated Denver International Airport CEO Phil Washington to serve as FAA administrator.

Transportation Secretary Pete Buttigieg confirmed Washington’s withdrawal in a tweet Saturday night, calling him "an excellent nominee" and blaming undeserved and partisan attacks.

"The partisan attacks and procedural obstruction he has faced are undeserved, but I respect his decision to withdraw and am grateful for his service," Buttigieg said on Twitter.

The FAA has lacked a Senate-confirmed administrator since March 2022. The agency is trying to reassure Americans that air travel is safe despite a surge in close calls between planes this year. It is also struggling with aging technology that failed in January, briefly canceling all takeoffs around the country. And it is still trying to repair its reputation after approving Boeing planes that crashed in 2018 and 2019.

The Senate Commerce Committee earlier this week delayed a vote on the nomination citing outstanding questions by some lawmakers.

Senator Kyrsten Sinema, a committee Democrat, had not announced whether she would support him and Senator Jon Tester also was still considering how to vote, a spokesperson said this week.

Senator Ted Cruz, ranking Republican on the Senate Commerce Committee, said late Saturday that it has been clear since his nomination that "Mr. Washington lacked the aviation experience necessary to run the FAA ... The Biden administration must now quickly name someone to head the FAA who has an extensive aviation background, can earn widespread bipartisan support in the Senate and will keep the flying public safe."

Cruz and other Republicans had said Washington, who retired from the U.S. Army in July 2000, needed a waiver from rules requiring civilian leadership to head the FAA. The Transportation Department's general counsel said Washington was fully qualified and did not need a waiver.

Cruz noted Washington has only about two years of experience as an airport CEO and criticized Washington's inability to answer some aviation questions at his confirmation hearing.

The White House insisted Washington was fully qualified. Cantwell had said he would shake up the agency, saying, "We feel that industry and FAA got too cozy."

A White House official had earlier told Reuters that, "Politics must not hold up confirming an administrator to lead the FAA, and we will move expeditiously to nominate a new candidate for FAA administrator."

The official said, "An onslaught of unfounded Republican attacks on Mr Washington’s service and experience irresponsibly delayed this process, threatened unnecessary procedural hurdles on the Senate floor, and ultimately have led him to withdraw his nomination today."

Washington was originally nominated in July but did not get a hearing from the Commerce Committee until March 1.

The FAA has had a number of recent safety issues.

In January, the FAA halted all departing passenger airline flights for nearly two hours because of a pilot messaging database outage, the first nationwide ground stop of its kind since the Sept. 11, 2001, attacks.

On Wednesday, the FAA issued a safety alert to airlines, pilots and others about the "need for continued vigilance and attention to mitigation of safety risks" after a series of high-profile near collisions.

Six serious runway incursions have occurred since January that prompted the agency to convene a safety summit last week.

Some industry officials think the White House could name acting FAA Administrator Billy Nolen as a new nominee. Nolen, who was named head of the FAA's aviation safety office, has been the acting FAA administrator since April 2022 and has received backing from many Republicans in Congress.

Washington had won support from a wide range of groups, including a number of aviation unions and a group of family members of some killed in a 2019 fatal Boeing 737 MAX crash.

The FAA has been without a permanent administrator for almost a year.

This was the second major Bide nominee to withdraw in recent weeks. Gigi Sohn, his pick for a key fifth seat on the Federal Communications Commission (FCC), withdrew dealing a setback for Democrats who have been unable to take control of the telecom regulator for more than two years.