Rosenstein sought to walk a fine line in his testimony before the Senate Judiciary Committee between defending the findings of Mueller's team on Russia and agreeing that some in the FBI committed wrongdoing.
"I decided that appointing a special counsel was the best way to complete the investigation appropriately and promote public confidence in its conclusions," Rosenstein said in his opening statement. "As we now know, the eventual conclusions were that Russians committed crimes seeking to influence the election and Americans did not conspire with them."
The former No. 2 at the Justice Department, who supervised the Mueller probe that came under constant attack from Trump, will acknowledge the problems with the Foreign Intelligence Surveillance Act warrants obtained on former Trump adviser Carter Page that were documented by the Justice Department inspector general.
"Senators, whenever agents or prosecutors make serious mistakes or engage in misconduct, the Department of Justice must take remedial action. And if existing policies fall short, those policies need to be changed," Rosenstein said.
Still, the FBI's Russia investigation is likely to be campaign fodder for the general election, as Trump's campaign and congressional Republicans have turned their attention to former Vice President Joe Biden, Trump's presumptive 2020 opponent.
Rosenstein's testimony is the first in a series of hearings that Senate Judiciary Chairman Lindsey Graham, a South Carolina Republican, is planning as part of a probe challenging the origins of the FBI's investigation into Trump's team and Russia, known as Crossfire Hurricane, and the Mueller probe, which followed it.
"We have looked at Russia's role in the election. Now we're going to look at the Mueller investigation. And we're going to look hard," Graham said. "We're going to be looking at the Mueller appointment in May of 2017, to see if there was a crime worthy of being investigated. ... We're going to be talking about how it got off the rails, who's responsible for it getting off the rails."
Graham and Senate Homeland Security and Governmental Affairs Chairman Ron Johnson, a Wisconsin Republican who has launched his own committee probe into the FBI's investigation into Trump's team, are holding separate votes Thursday that would give them wide-ranging subpoena power for documents and testimony of top Obama administration officials.
Still, Democrats may press Rosenstein, too, about authoring a different memo used to justify the firing of then-FBI Director James Comey, as well as Rosenstein's role in the Justice Department's conclusion that there was not sufficient evidence in the Mueller investigation to charge Trump with obstruction of justice.
Democrats have dismissed the investigations as an election-year effort to boost Trump's reelection bid and rewrite the history of the Mueller investigation.
"Contrary to the President's claims, IG Horowitz found no evidence of political or anti-trump bias in the Crossfire Hurricane investigation," said Sen. Dianne Feinstein of California, the top Democrat on the Senate Judiciary Committee. "IG Horowitz confirmed that none of the FISA errors his investigation uncovered call into question 'any part of the special counsel's report.'"
Rosenstein, who is testifying voluntarily, was not present for the start of the Russia probe in July 2016, but he signed off on the third renewal of a FISA court warrant approved for Page -- warrants that the inspector general concluded were undermined by significant problems.
Rosenstein told the inspector general he did not recall the details of the briefings he had received on the Page warrant or those he was given about the opening of the Crossfire Hurricane investigation. He said he "did not have an opinion about" whether there was sufficient information to open the probe.
"I'd like to know from him, was he misled?" said Sen. Josh Hawley, a Missouri Republican on the Judiciary panel. "Let's find out what he did, and what he actually was briefed on, what he was aware of."
Many of the major events of the Mueller investigation happened on Rosenstein's watch, so Wednesday could become a comprehensive revisiting of the FBI's and the Mueller team's work.
Rosenstein was the top Justice Department official overseeing the indictments of Russians for election interference and the major obstruction and conspiracy prosecutions of Trump advisers, including former campaign chair Paul Manafort and his deputy Rick Gates, the President's personal attorney Michael Cohen, longtime friend Roger Stone and Flynn.
It was Rosenstein's directive in Mueller's initial months, too, that outlined how the special counsel should investigate whether Page, Manafort and foreign-policy campaign adviser George Papadopoulos colluded with Russian government officials. Mueller's team did not find evidence of any conspiracy between Trump's associates and Russia, but he did conclude that those inside and associated with the Trump campaign had welcomed and encouraged Russian activity they thought could help Trump win.
Rosenstein also became a major witness to the other half of the Mueller investigation: documenting the President's attempts to obstruct the probe. Trump was not charged with obstruction after Barr and Rosenstein reviewed the Mueller report, Barr told Congress after he received Mueller's findings.
This story has been updated with additional developments Wednesday.