Later, Rosenstein gave into those demands and apointed Robert Mueller, the FBI Director for 12 years before Comey, as the special counsel to conduct the investigation into Russian meddling and any ties to the Trump presidential campaign.
Mueller, who became FBI Director a week before the September 11, 2001, terrorist attacks, guided the bureau into a war-time footing focused on protecting the homeland.
In 2011, President Barack Obama asked for and received a 100-to-0 vote to grant a two-year extension of the usual 10-year term granted to FBI directors.
Democrats were quick to applaud Rosenstein's appointment of Mueller, who became a close ally of Comey's while he served as deputy attorney-general in 2003 and 2004. In one dramatic standoff, Mueller, Comey and other senior Justice Department officials threatened to resign en masse over a dispute related to warrantless wiretapping.
But Rosenstein will still face tough questions from Democrats and Republicans who want to understand the timeline of events over the past nine days.
"We'll have a chance to ask him about the scope of Sessions recusal, about the Comey firing, about his memo," Democrat Senator Claire McCaskill said before the Mueller announcement.
Senator Jeff Flake, a member of the Judiciary Committee, said he wants to know if Rosenstein had seen or heard about Comey's interactions with the president - which have come to light in notes that the FBI Director filed away after each meeting - before Rosenstein wrote his analysis of Comey's FBI tenure that was released upon his dismissal.
"Obviously people are going to want to know if he got that memo before he wrote his memo, if he got his Comey memo before he wrote his Comey memo. That's the big question," Flake said.
The Mueller appointment was greeted with a deep sigh of relief among Republicans. Each day, with each new revelation, more Republicans came closer to calling for a special counsel but usually with a caveat of "maybe" the time had come for such a step. They were hesitant to call for such an investigation of a president from their own party, but they were weary from being on the defensive as Trump allegations mounted.
Now, with such a respected appointee overseeing the investigation, Republicans have a ready-made defence to shield themselves from inquiries: Bob Mueller is going to get to the bottom of this.
Today marked the third straight on Capitol Hill where late-breaking news caused whiplash as lawmakers came to the chamber to vote.
On Tuesday, it was the Washington Post's report that Trump, in an Oval Office meeting last week, provided to Russian diplomats highly classified information gleaned from a then-undisclosed ally about Isis.
Yesterday, it was the New York Times report that in mid-February, after Trump was forced to dismiss his national security adviser, Michael Flynn, over lying to superiors about his contacts with Russia, the President pulled Comey aside and asked him to "let it go" regarding the ongoing investigation of Flynn.
That report sent senators into a frenzy during yesterday's round of votes, when they could be seen anxiously chatting among themselves. Democrat Senator Heidi Heitkamp saw them too - and realised she had missed out on something.
"What now?" she screamed across the chamber.
McCaskill walked over and briefed Heitkamp on the latest breaking news.
Regardless of the Mueller appointment, senators will want Rosenstein's assurance that all of Comey's memos about Trump will be provided to the relevant committees overseeing investigations. "I want to see the evidence," she said. "I don't want to just read about it. I want to see it."
Rosenstein's shaky standing is a stunning turnaround. In 2005 President George W. Bush nominated him to be US attorney for Maryland. He won such high praise that the state's Democrats convinced President Barack Obama to keep him on in 2009.
During his confirmation hearing, Rosenstein promised to be an independent voice in considering such a move. "I would evaluate the facts and the law [and] consider the applicable law," he told the Senate Judiciary Committee.
But two weeks after Rosenstein's confirmation, Trump dismissed Comey. His aides cited Rosenstein's memo as the reason for his firing - until Trump himself cited the "Russia thing" in an NBC News interview two days later.
Rosenstein's previous supporters now question whether they ever should have supported him. "I have real questions about his judgment," Coons said, "given the memo, the memo that was cited as the reason for the firing FBI Director Jim Comey, and the subsequent shifting White House justification for why the FBI director was fired needs to be more thoroughly explained."
His appointment of Mueller is the first step to winning back their trust.
