The U.S. Defense Department is an $849 billion enterprise with close to three million employees — 1.3 million of them active-duty service members — and 750 military bases around the world. Its stated mission is to provide “the military forces needed to deter war and ensure our nation’s security.”

So President-elect Donald J. Trump’s decision to nominate Pete Hegseth to lead the institution hit immediate headwinds from an array of lawmakers and senior military officials. Mr. Hegseth, a Fox News host, is a veteran of the wars in Iraq and Afghanistan, but he has no senior command experience.

“This is not an entry-level job,” said Representative Jason Crow, Democrat of Colorado and a retired Army Ranger, who expressed his opposition to Mr. Hegseth running the Pentagon in a video. “This man is woefully unqualified to make the types of life-and-death decisions the secretary of defense has to make.”

Even some Senate Republicans expressed doubts.

“He’s got his work cut out for him,” Senator Joni Ernst, an Iowa Republican who was believed to have been on Mr. Trump’s short list for the defense secretary job, said of Mr. Hegseth on Wednesday.

Senator Todd Young, Republican of Indiana, said he was excited about many of Mr. Trump’s nominees. But when it came to Mr. Hegseth, he said, “I don’t know much about his background or vision.”

Democratic lawmakers — even those who often reach across the aisle on national security issues — were more blunt.

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