Former Department of Defense general counsel Jeh Johnson testifies Wednesday before the Senate Homeland Security and Governmental Affairs Committee during his confirmation hearing to be the next secretary of homeland security on Capitol Hill. (Chip Somodevilla/Getty Images)

Jeh C. Johnson said Wednesday that his first priority if confirmed as secretary of homeland security will be to quickly fill the large number of senior management vacancies at the agency.

Johnson also promised to boost morale at the sprawling 240,000-employee department, improve financial management, and be transparent with Congress and the public about programs and policies.

“If confirmed, I pledge to be a champion for every man and woman of the Department of Homeland Security and their families,” Johnson said at a largely friendly confirmation hearing before the Senate Homeland Security Committee. “I hope to be a visible leader [and] remind people of the importance of the overriding, unifying mission of homeland security.”

Johnson, 56, the Pentagon’s former general counsel, was nominated by President Obama last month to succeed Janet Napolitano.

Johnson, who has passed Senate muster twice before — for the Pentagon job and as Air Force general counsel — is expected to be confirmed easily. The committee received letters of support for Johnson from all three former DHS secretaries.

But he will take on the daunting challenge of building cohesion at a department cobbled together from 22 agencies with missions as diverse as enforcing immigration laws, securing the borders, countering terrorism and preventing cyberattacks.

About 40 percent of its senior leadership positions are vacant or have an “acting” place-holder. Its employees on average have the lowest morale of any federal agency, according to Congress’s auditing arm. The Government Accountability Office has also routinely placed DHS on its “high-risk” list of agencies vulnerable to waste and mismanagement.

Johnson stressed his background in helping shape the Obama administration’s counterterrorism policies as one reason he was well positioned to lead the department.

Increasingly, he said, as the core of al-Qaeda has been diminished, “the terrorist threat is becoming even more diffuse, and we’re seeing more lone-wolf activity, more self-radicalization.” Those types of threats, he said, are harder to detect.

As a result, he added, “we’re going to have to be vigilant on the civilian side in law enforcement, border security and so forth.”

In an exchange with Sen. Kelly Ayotte (R-N.H.), Johnson said he believes that U.S. officials have the legal authority to question terrorism suspects before they enter the criminal justice system. He said he also thinks that Congress and the executive branch ought to look at writing this authority into law, “because I think it’s going to become an increasingly important practice and there will be increasing need for this type of interrogation.”

With the advent of drones for surveillance, including along the nation’s borders, Johnson said he recognized the need to respect Americans’ privacy rights. “As we move to more advanced technology, I think we also need to be sensitive to privacy, civil liberties concerns that people who live along these borders may have,” he said.

In perhaps the testiest exchange of the hearing, Sen. John McCain (R-Ariz.) repeatedly pressed Johnson to promise to tell the committee what is needed so that Customs and Border Protection could have “90 percent” effective control of the southern border. Johnson said he wanted to better understand the issue before committing to an assessment. McCain said he could not support Johnson’s confirmation until he got such a commitment.

Sen. Rand Paul (R-Ky.) asked Johnson whether he thought the United States should target for killing American citizens overseas who are not involved in combat. He said he was thinking of “propagandists” or people who may have committed treason but have not been charged.

“Do you think that a bunch of lawyers in a room from one administration, from one political party, can decide the guilt or innocence of American citizens?” Paul asked.

“As you pose it, I think my answer would be no,” Johnson said.

Johnson said he prided himself in challenging assumptions made by others in planning counterterrorism operations. “Every once in a while, I felt like it was 11 to one, and I was the one. And I said to myself, ‘This is why the president put you here.’ ”