William Barr, President Donald Trump's attorney general nominee, would let special counsel Robert Mueller complete his ongoing investigation of Russia's election meddling and possible collusion with Trump's 2016 campaign, Sen. Lindsey Graham told reporters Wednesday.

"I can assure you he has a very high opinion of Mr. Mueller and he is committed to letting Mr. Mueller finish his job," Graham, R-S.C., said after a meeting with Barr in his Capitol Hill office.

The reassurance from Graham, who is one of Trump's most vocal supporters, came amid reports that Deputy Attorney General Rod Rosenstein, who appointed Mueller, is planning to voluntarily step down from his No. 2 role at the Justice Department within the next month. NBC News, citing administration officials, reported that Rosenstein had intended to stay in the role for about two years.

Reports of Rosenstein's forthcoming resignation fueled speculation that Mueller's probe could perhaps be nearing its long-anticipated end, even though a federal judge recently approved an extension of Mueller's grand jury by up to six months.

Some have even taken the reports as a signal that Rosenstein believes the investigation would not be threatened under Barr's oversight — an interpretation that could offer relief to Mueller's supporters, some of whom have criticized Barr's past remarks on the Russia probe.

In June, Barr reportedly sent a 19-page memo to the DOJ arguing that Mueller's focus on whether Trump had obstructed justice "is fatally misconceived."

Public theories about the obstruction facet of the investigation have largely zeroed in on Trump's decision to fire then-FBI Director James Comey in May 2017, after Trump allegedly said he should let go of a criminal investigation into Trump's first national security advisor, Michael Flynn. Flynn's sentencing on a charge of lying to the FBI about his conversations in 2016 with then-Russian Ambassador Sergey Kislyak, originally set for December, was postponed until at least March.

But Barr gave no indication of his past criticisms of Mueller on Wednesday. And en route to the office of Republican Sen. Ben Sasse of Nebraska after his meeting with Graham on Wednesday, Barr told reporters that he believed Rosenstein has done an "excellent" job as deputy attorney general.

Graham's role in tamping down concerns about Trump's pick to lead the Justice Department offered another display of the influence Graham still maintains in the upper echelons of the Trump administration.

He played a nearly identical role amid a barrage of criticism over the selection of former U.S. Attorney Matthew Whitaker to succeed fired Attorney General Jeff Sessions in an acting role.

Like Barr, Whitaker had been fiercely critiqued by Democrats and watchdog groups for his previous criticism about the special counsel. Whitaker also came under intense scrutiny for his association with a patent company that was accused of defrauding its clients. And his appointment to the acting role without going through the Senate confirmation process was decried by some legal scholars as unconstitutional.

In November, Graham took Whitaker into his office for a closed-door meeting, and emerged with confidence that Whitaker, who had reportedly taken oversight control of the Mueller probe from Rosenstein, had no intention to obstruct or quash the Russia probe.

"I don't think he's going to do anything unsavory," Graham told reporters after that meeting.

Graham remains a close ally and supporter of Trump, despite his disappointments with the president's response to the killing of Washington Post journalist Jamal Khashoggi in the Saudi Consulate in Istanbul. Graham has widely been credited as the difference-maker in the successful appointment of Trump's second Supreme Court nominee, Brett Kavanaugh.

Barr is scheduled to face two days of confirmation hearings before the Senate Judiciary Committee beginning Tuesday.