WASHINGTON – U.S. Air Force Gen. Terrence O’Shaughnessy, the current commander of Pacific Air Forces, is the leading nominee to become the next head of U.S. Pacific Command – a position historically held by Navy officials, Defense News has learned.

Four sources with knowledge of the discussions said O’Shaughnessy is the leading candidate for the spot, which would mark the first time in history the PACOM spot has not been filled by the Navy – a potential roadblock for Sen. John McCain, the chairman of the Senate Armed Services Committee.

Notably, O’Shaughnessy is well regarded by Adm. Harry Harris, the current PACOM head who has emerged as a popular figure with the Trump White House. Harris is expected to retire next year.

O’Shaughnessy is a well-known figure to those in the Pacific theater. Among his assignments, he spent 2012-213 as deputy director for politico-military affairs for Asia on the Joint Staff, followed by 14-month stint as director for operations – headquarters with PACOM and a two-year stint as the deputy commander of U.S. forces in Korea.

One source familiar with O’Shaughnessy described the F-16 pilot as a “humble” guy who has largely avoided the spotlight, which could help his cause when it comes time for a formal nomination.

The biggest hurdle for the nomination is likely to come from McCain, the Arizona Republican who is seen as friendly towards the Navy and historically critical of the Air Force.

The Air Force has not briefed SASC staffers about the O’Shaughnessy nomination, which would be the first step before sitting down with McCain, an Air Force source told Defense News.

The service is hesitant to confront McCain with the information because of tensions between the SASC chairman and top administration officials — which have led to him blocking nominations from moving forward — and McCain’s longstanding hostility toward the Air Force.

Although the McCain’s relationship with the Air Force is improving, it’s “totally unclear” if he will be supportive of O’Shaughnessy’s nomination, the source said.

Inside the Navy, the race for the PACOM nomination was shaken up when the current head of U.S. Pacific Fleet announced in late September that the chief of naval operations, Adm. John Richardson, told him he was being passed over. He was pulled from contention after a summer that saw two Pacific Fleet ships colliding with commercial vessels, killing 17 sailors and causing hundreds-of-millions in damages.

The Navy’s likely nominee would be Fleet Forces Commander Adm. Phil Davidson. Davidson has little experience in the Pacific Theater; his career is largely based on the East Coast and in the Middle East.

Adm. Scott Swift had long been the predicted nominee for the PACOM spot, but announced plans to retire in September, citing the fact he had been told he would not be given the job.

“We think that Davidson is a sound nominee,” said a Navy source familiar with the internal discussions. “I think we realized that Swift was unconfirmable.”

Davidson has previously commanded the Eisenhower Carrier Strike Group, the cruiser Gettysburg and the frigate Taylor. His shore assignments include deputy director for strategy and policy in the Joint Staff; senior military advisor to the Special Representative for Afghanistan and Pakistan at the State Department, and a previous tour at Fleet Forces Command.

Before taking Fleet Forces, he was the commander of the U.S. 6th Fleet in Europe. He has previous experience earlier in his career at a staff job at PACFLT, according to his official biography.

“Davidson is by no means a throw-away candidate,” the source said. “That said, we realize that O’Shaughnessy is a strong pick, and if he gets the nod instead of Davidson, we’ll look to place Davidson elsewhere.”

Davidson is also said to be a strong contender for vice chief of naval operations if he is passed up for PACOM. The current vice chief, Adm. Bill Moran, is a leading contender to replace Swift when he retires from PACFLT.