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Mr. Johnson steps into several thorny issues, including immigration enforcement.

Mr. Obama is under pressure to halt most deportations, just as he did last year for most young illegal immigrants.

“We hope Secretary Johnson will begin immediately to reverse course on the morally indefensible policies implemented by his predecessor,” said Pablo Alvarado, executive director of the National Day Laborer Organizing Network, calling on Mr. Johnson to reject what he said was the department’s “quota” to deport 400,000 people each year — the figure officials say is what is budgeted.

Republicans, who oppose halting deportations, have tried to prod Mr. Johnson to say what he will do. In a letter last month, a handful of Republican senators sent him a letter asking him whether he would see through Ms. Napolitano’s nondeportation policies.

In response, Mr. Johnson said he was unable to respond to the specific queries question by question. In general, he said, he believes the department does have “prosecutorial discretion” powers to decide whom to target for deportations and can issue memos ordering employees how to follow through, though the department cannot refuse to enforce laws altogether.

The Homeland Security Department, created in the wake of the Sept. 11, 2001, terrorist attacks, has chief responsibility for cybersecurity.

A recent inspector general report warned that coordination among the department’s 22 agencies and offices remains a major challenge. Ms. Napolitano left three months ago to become president of the University of California system. Without mentioning her by name, the report points to a culture of “ineffective management” and “cost inefficiencies” that continues to plague the agency.

The department’s biggest problem areas as outlined by the report include management of cybersecurity, transportation security, border security and infrastructure protection.

Mr. Carper did point out Monday that last week, the department received a clean financial audit for the first time.