President Trump’s choice to take the No. 2 job at the Pentagon had a rocky confirmation hearing Tuesday, with Senate Armed Services Committee Chairman John McCain (R-Ariz.) at one point threatening to withhold his nomination from a vote and other lawmakers questioning how he will overcome his lack of experience in the Defense Department.

Patrick M. Shanahan, a vice president at the aerospace company and defense contractor Boeing, who was nominated in March to be deputy defense secretary, also faced questions about how he would manage day-to-day operations in the Pentagon while recusing himself from all decisions with a tie to Boeing. Shanahan has worked for the defense behemoth since 1986, with stints overseeing civilian airliner programs and military equipment.

McCain needled Shanahan early in the hearing about his prepared answer to a question about the U.S. potentially supplying weapons to Ukraine to face Russian-backed separatists. Shanahan wrote that he would have to look at the issue.

“In your questions that were submitted to you, one of the questions was providing the Ukrainians with legal, lethal defense weaponry with which to defend themselves,” McCain said. “Inexplicably, you responded by saying you have to look at the issue. It’s not satisfactory, Mr. Shanahan.”

The nominee, asked whether he wanted to amend his answer, quickly responded that he supported the idea. But McCain continued his line of questioning, saying he found Shanahan’s answer “very disappointing to me,” especially considering his years of work on weapons programs with Boeing.

“That’s not good enough, Mr. Shanahan,” McCain said. “I’m glad to hear you changed your opinion from what was submitted, but it’s still disturbing to me. It’s still disturbing to me after all these years that you would say that you would have to look at the issue. Have you not been aware of the issue? Have you not been aware of the actions of the Senate Armed Services Committee? Have you not been aware of the thousands of people that have been killed by [Russian President] Vladimir Putin?”

McCain said that if Shanahan chose not to respond directly to a question again, he would not bring his nomination to the committee for a vote. Shanahan responded that he was “very clear” about that.

“I think the Russians are adversarial,” Shanahan said. “I think through the whole of government, we need to deal with their [actions] — whether we call it aggression or their disruption to our interests. I, at this point, don’t have any specific recommendations. If confirmed, I will spend a significant amount of time dealing with Russia.”

On his lack of experience in the Defense Department, Shanahan said he has worked in a variety of organizations and thinks his technical and management experience “will prepare me to be able to quickly assimilate the knowledge and expertise to properly interface.”

Shanahan answered questions in greater detail about weapons acquisition and procurement, saying that “this is an area where I’ve had some fairly good success” and that rather than try to overhaul the entire system, he would focus on specific problem areas. By scrutinizing how technology prototypes perform in testing, “we could demonstrate what works” and then replicate the process, he said.

“It’s in doing those prototypes that you can get a quick win,” he said. “And then you also find out where the real limitations in the system are.”

Shanahan was introduced by Sen. Maria Cantwell (D-Wash.), whose state is home to numerous Boeing facilities. She lauded his past efforts to drive change at Boeing and said his attention to detail will prove invaluable in the Pentagon.

“He is also fearless,” she said. “He understands what our country is up against when it comes to the Russians and the Chinese and the North Koreans, and it won’t faze him. He focuses on big, game-changing innovation in science and technology and won’t be deterred by the bureaucracy of DOD.”

Shanahan said his experience in industry and innovation has prepared him to contribute as deputy defense secretary and will help him complement Defense Secretary Jim Mattis, whom he called a “master strategist with deep military and foreign policy experience.”

Added Shanahan: “I bring with me a formula for leadership that has a record of delivering affordable high-performing business systems and operations under adverse conditions. Leadership casts a long shadow, and strong leadership can create teams that achieve ambitious change of scale.”

If confirmed, Shanahan will replace Deputy Defense Secretary Robert O. Work, a retired Marine colonel who held the position under President Barack Obama and was asked by Mattis to stay on for several months in the new administration.