Rod J. Rosenstein, the U.S. attorney in Baltimore and the longest-serving U.S. attorney, is the incoming Trump administration’s pick to become the next deputy attorney general — the second-highest position in the Justice Department, according to a member of the Trump transition team.

Rosenstein, 52, is the sole holdover U.S. attorney from the George W. Bush administration who is still in office. He would replace Deputy Attorney General Sally Q. Yates, the former U.S. attorney from Georgia.

As the deputy attorney general, Rosenstein, who must be confirmed by the Senate, would be responsible for the day-to-day running of the sprawling department of about 113,000 employees with its 94 districts, essentially becoming its chief operating officer. The heads of several agencies, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, would report to him. His nomination was first reported by CNN and confirmed by the member of President-elect Donald Trump’s transition team, speaking on the condition of anonymity because the matter has not been officially announced.

Rosenstein became U.S. attorney for the District of Maryland in 2005 and is widely credited with calming what was then an office in turmoil. Poor management had undermined morale within the office and hurt relations with the FBI after the Sept. 11, 2001, attacks, current and former colleagues said.

Colleagues say he is able to withstand political pressure from any quarter — including the White House.

“Rod is not someone who would stand silent in the face of something he believed was wrong,” said Jason M. Weinstein, a former federal prosecutor who was Rosenstein’s violent crimes chief and later deputy assistant attorney general of the criminal division. “He has the courage of his convictions, and he is committed to doing the right thing for the right reasons.”

Kevin Perkins, the former head of the FBI field office in Baltimore, said: “Rod is a steady hand, and he doesn’t seek the limelight. He really turned that office around in a short amount of time.”

Rosenstein’s office successfully prosecuted Jack Johnson, then Prince George’s county executive, for corruption. Johnson was sentenced to seven years in prison in 2011. He also oversaw the investigation of former vice chairman of the Joint Chiefs of Staff Gen. James “Hoss” Cartwright, who pleaded guilty in October to one felony count of lying to the FBI in a classified leak probe.

His office is handling the prosecution of Harold Thomas Martin III, a former National Security Agency contractor accused of stealing huge quantities of classified information and storing them in his house and car.

Rosenstein also started a national security section in his office, which has more than 80 assistant U.S. attorneys. In October, the section obtained an indictment on Nelash Mohamed Das, a Bangladeshi citizen living in Landover who is accused of planning to kill a U.S. military member in support of the Islamic State.

Weinstein, now a partner at Steptoe & Johnson, said that Rosenstein pursued public officials for corruption without regard to party. “He was apolitical in his pursuit of officials.”

Rosenstein, a Harvard law graduate, joined the Justice Department in 1990 as a trial attorney in the criminal division’s public integrity section.