But Sen. Chris Van Hollen, D-Maryland -- who introduced Rosenstein during his Senate confirmation hearing, recommending him for the post -- said Wednesday he's "disappointed" with Rosenstein's letter to Trump.
"I read his letter, and to me, it did not pass the credibility test," Van Hollen said.
Because Sessions recused himself from the Russia investigation, Rosenstein is currently the highest-ranking DOJ member leading the probe.
Since the firing, reports have surfaced that Comey was removed from his post just one day after he had asked the Department of Justice for more resources to aid in his investigation of the Trump campaign’s possible ties to Russia during the 2016 presidential election -- but the DOJ has denied that this happened.
Still, Van Hollen believes the timing of Comey's firing is suspect.
"This is all about the timing, and the timing of this really smells," Von Hollen said.
But it's the White House that's to blame for the upset, he said. He noted that Comey's firing came around the same time that reports surfaced that grand jury subpoenas were sent to former national security adviser Michael Flynn's associates. Van Hollen also pointed out that the firing came days after Comey testified in Congress that he was proceeding with the investigation.
"It looks to me like as this investigation began to heat up ... the hammer came down on Comey," Van Hollen said. "If the White House has nothing to hide here, why not allow this to play out?"
When asked if Trump's actions might constitute an impeachable offense, Van Hollen claimed there are many indicators that the Trump campaign may have collaborated with the Russians, but that it would be helpful to bring Rosenstein and Sessions to testify before the Senate.
"All those lines have not been connected," he said. "And the reason for bringing both the attorney general and the deputy attorney general before the Senate is to see whether there's a connection between those building blocks."
"It is really difficult to believe the idea that the person who led the chants of 'lock her up!' was really worried about how the FBI handled the email investigation," he said.