The deputy attorney general, who's been in law enforcement for close to three decades, is back in the news after the President tweeted Friday morning that he is being investigated for his role in firing former FBI Director James Comey and seemed to accuse Rosenstein of overseeing a "witch hunt" against him.
"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director!" Trump said on Twitter, calling the probe into him a "Witch Hunt."
Rosenstein is at the center of Comey's departure: His memo to Attorney General Jeff Sessions is the basis that White House officials and Trump himself have used to defend Comey's dismissal, despite the FBI's ongoing investigation into the 2016 campaign and Russia's attempts to help Trump win.
"The way the director handled the conclusion of the email investigation was wrong," Rosenstein wrote, failing to acknowledge the fact Trump cheered Comey's inquiry of Clinton on during the 2016 campaign. "As a result, the FBI is unlikely to regain public and congressional trust until it has a director who understands the gravity of the mistakes and pledges never to repeat them."
He added: "Having refused to admit his errors, the director cannot be expected to implement the necessary corrective actions."
Rosenstein, 52, has kept a relatively low profile throughout his decades working inside the Justice Department, a fact that likely helped him win bipartisan approval weeks ago when 94 senators voted to confirm his appointment to deputy attorney general. Only six lawmakers voted no.
His memo criticizing Comey -- issued amid the former FBI director's investigation into Russia's 2016 actions -- has cast doubt over his objectivity in the eyes of many Democrats.
Democrats challenged Rosenstein on whether he would appoint a special prosecutor to look into Russia's election meddling. Rosenstein last month appointed former FBI Director Robert Mueller
as special counsel overseeing the probe.
Rosenstein addressed dealing with political considerations around investigations during an interview with ABC2 News in Baltimore earlier this year, arguing that "it is not difficult" for him to separate politics and the way he prosecutes cases.
"It is important to separate the roll of politics in setting priorities and the decision to prosecute cases. And in the Justice Department that is what we do on a daily basis, that is how are trained. So, it is not difficult for me," he said, arguing that while a new president and attorney general will have new priorities, the way the department peruses and prosecutes cases doesn't change.
"As deputy attorney general, you are move involved in policy making, but in terms of the principles of the department and how we conduct our investigations and who we prosecute, those decisions are going to be made in the same way they have always been made," he said.
Lengthy DOJ career
Rosenstein, born in Pennsylvania, joined the Justice Department in 1990 and moved up the organization over the span of 27 years.
After rising through the ranks, Rosenstein was unanimously confirmed as United States attorney for the District of Maryland in 2005 under then-President George W. Bush.
But Rosenstein's rise through Justice has not been without pitfalls.
The Bush administration nominated Rosenstein to the federal appeals court in Richmond in 2007, but his nomination was blocked by Maryland's two Democratic senators at the time, Barbara Mikulski and Benjamin Cardin, who argued they needed Rosenstein in Maryland, not Virginia.
"In the twilight of the Bush administration, we don't need an acting US attorney in Maryland," Mikulski said at the time.
When President Barack Obama entered office, Rosenstein was asked to stay on and served for all eight years of the president's term.
The feat makes him the only Bush-appointed US attorney to serve all eight years under Obama.
His time under Bush, Obama and then Trump made him the longest-serving US attorney in the nation's history when he was confirmed to his current role under Sessions.
In an attempt to inoculate the Trump administration from criticism over Comey, many top officials have heralded Rosenstein.
"He is a man of extraordinary independence and integrity ... and great character," Vice President Mike Pence told reporters on Wednesday, noting that the longtime Justice Department official "came to work, sat down and made his recommendation" to fire Comey.
"He brought that recommendation to the President" and Trump agreed, Pence concluded.