The Justice Department appointed Robert S. Mueller III, a former F.B.I. director, as special counsel on Wednesday to oversee the investigation into ties between President Trump’s campaign and Russian officials, dramatically raising the legal and political stakes in an affair that has threatened to engulf Mr. Trump’s 118-day-old presidency.
The decision, by the deputy attorney general, Rod J. Rosenstein, came after a cascade of damaging developments for Mr. Trump in recent days, including his abrupt dismissal of the F.B.I. director, James B. Comey, and the subsequent disclosure that Mr. Trump asked Mr. Comey to drop the investigation of his former national security adviser, Michael T. Flynn.
Mr. Rosenstein, who wrote a memo that the White House initially cited as the rationale for Mr. Comey’s dismissal, had been under escalating pressure from Democrats, and even some Republicans, to appoint a special counsel.
By appointing Mr. Mueller, a former federal prosecutor with an unblemished reputation who once stood up to President George W. Bush on the legality of his domestic wiretapping program, Mr. Rosenstein could alleviate questions about the government’s capacity to investigate the swirl of questions surrounding the Trump campaign and the Russians.