WASHINGTON (AP) - President Barack Obama's selection of a former top Pentagon lawyer to head the Homeland Security Department suggests the agency will be stepping back from its preoccupation with immigration to focus more on protecting the nation from attack. Jeh C. Johnson, if confirmed by the Senate, would replace Homeland Security Secretary Janet Napolitano, who left the DHS last month to become president of the University of California system. Obama was expected to announce Johnson's nomination Friday. Unlike Napolitano, Johnson has spent most of his career dealing with weighty national security issues as a top military lawyer. Issues he handled included ending the military's don't-ask-don't-tell policy for gay service members and changing military commissions to try terrorism suspects rather than using civilian courts. He also oversaw the escalation of the use of unmanned drone strikes during the wars in Iraq and Afghanistan as general counsel at the Defense Department. Napolitano, who came to the DHS after serving as governor of Arizona, made clear that her top priority was immigration reform and routinely championed the issue in congressional testimony. During her first hearing on Capitol Hill, she did not mention terrorism. That is unlikely to be the case with Johnson, who left the Defense Department in 2012 and previously served as the general counsel of the Air Force under President Bill Clinton. Earlier in his career, he served as an assistant U.S. attorney in the Southern District of New York, where he handled public corruption cases. Obama repeatedly has declared immigration reform a top priority, saying as recently as Thursday that he will push for a landmark overhaul bill this year. Based on Johnson's resume, which does not include immigration issues, that means the White House likely would lead the push for legal changes. The Homeland Security Department's responsibilities include routine immigration issues, cybersecurity, protecting the president and keeping would-be terrorists off airplanes. The DHS also includes the Coast Guard. Tom Ridge, the first DHS secretary and a former Pennsylvania governor, said this week that whoever takes over the department should be expert in a few of the department's missions, but not all. He said understanding national security is paramount. "We need someone who appreciates that you can't secure the country from inside the Beltway," said Ridge, who has been critical of some of Obama's national security strategies. Two previous secretaries, Napolitano and Michael Chertoff, were also one-time federal prosecutors. But Johnson's national security and military experience may eclipse both of them. Johnson led a widespread crackdown on unauthorized leaks in the Defense Department, including warning former Navy SEAL Matt Bissonnette that by publishing a book on the SEALs' raid that killed bin Laden, he was in "material breach" of two nondisclosure agreements he signed earlier in his career. Johnson also was involved in the investigation of retired Marine Gen. John Allen and Tampa, Fla., socialite Jill Kelley. Johnson sparked criticism when he said in a speech at Oxford University last November that the war on terror was not an endless conflict and that the U.S. was approaching a "tipping point" after which the military fight against al-Qaida would be replaced by a law enforcement and intelligence operation. Johnson would be the third black member of Obama's current Cabinet. He would join Attorney General Eric Holder and Transportation Secretary Anthony Foxx. The president was criticized earlier this year for not having more blacks in his Cabinet. Johnson would take over an agency with numerous high-level vacancies, including the deputy secretary. When Napolitano left, one-third of the heads of key agencies and divisions were filled with acting officials or had been vacant for months. Obama has nominated several people to key positions, including general counsel. His pick to be the department's No. 2, U.S. Citizenship and Immigration Services Director Alejandro Mayorkas, is the subject of an internal investigation, and his nomination has been stalled. Lawmakers applauded the selection of Johnson. "This nomination comes at a critical time for the agency and its mission because, for the past several months, the department has been operating without a Senate-confirmed secretary or deputy secretary, and also has numerous other high-level vacancies," said Sen. Tom Carper, D-Del., who chairs the Senate Homeland Security and Governmental Affairs Committee. After leaving the administration in 2012, he returned to private practice. Johnson earned more than $2.6 million from his partnership income at that law firm, according to 2009 government financial disclosure documents. During the 2008 presidential campaign, Johnson donated more than $33,000 to Obama's campaign, federal records show. He was also a supporter of Hillary Rodham Clinton, having contributed $2,300 to her presidential primary campaign in July 2008. He's also given $5,000 to the New Jersey Democratic Party and $1,000 to Democrats nationwide, as well as to several congressional candidates. Obama's campaign website listed Johnson as a member of the then-candidate's national finance committee and an adviser to Obama's foreign policy team during the 2008 election. Associated Press writers Nedra Pickler, Lolita C. Baldor and Jack Gillum contributed to this report.