Sen. Chuck Grassley (R-Iowa) has emerged as a pivotal player in the controversy surrounding President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE and Russia.

As chairman of the Senate Judiciary Committee, Grassley has threatened to subpoena Trump family members and associates over their roles in the controversy surrounding Russia’s involvement in last year’s presidential election.

He has also defended Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE, whom he had privately urged to recuse himself from the Justice Department’s investigation of Russian meddling given the former Alabama senator’s role in Trump’s campaign.

Grassley warned the White House that there will be no confirmation hearings this year on a new attorney general if Sessions is dismissed.

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In June, Grassley criticized the White House Office of Legal Counsel for an opinion holding that only committee chairs have the constitutional authority to make official inquiries of the executive branch, not rank-and-file members.

Grassley shot that down emphatically.

“This is nonsense,” he wrote in a letter to Trump.

The actions highlight Grassley’s reputation as a dogged investigator — and an independent streak that has led him to embrace government whistleblowers.

“He’s got a strong sense of right and wrong, and I think that sense even surpasses party, which around here is a real virtue,” said Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (Calif.), the ranking Democrat on the Judiciary Committee.

Grassley and Feinstein displayed notable cooperation in their efforts to secure testimony from Paul Manafort, Trump’s former campaign chairman, and Donald Trump Jr., the president’s eldest son.

Committee staff are negotiating to sit down with both men. The committee is expected to seek information from Jared Kusher, Trump’s son-in-law and a senior White House adviser, as well.

“This is vintage Chuck Grassley Charles (Chuck) Ernest GrassleySenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE. He believes in strong oversight and he’s going to be as aggressive as he feels like he needs to be in order to get it done,” said Senate Republican Whip John Cornyn John CornynBipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection Airline job cuts loom in battleground states MORE (Texas).

“Chuck Grassley has a reputation for rigorous oversight unmatched perhaps by anybody else in the Senate.”

Grassley’s committee is also trying to obtain memos that former FBI Director James Comey wrote of his interactions with Trump before his firing. His staff has reviewed the memos but doesn't possess them.

Grassley said in an interview that no one at the White House or within the Senate GOP leadership has tried to dissuade him.

“I’ve had no face-to-face contact with anybody, I’ve had no phone calls from anybody, not only my colleagues but not from the White House either,” he told The Hill.

Grassley has made investigations and oversight a priority throughout his career. As the Judiciary Committee chairman, he funnels a chunk of his resources into a seven-member team that is tasked almost exclusively with investigations — a rare set-up on Capitol Hill.

He operated a similar investigative unit with committee resources when he chaired the Senate Finance Committee.

One of his most high-profile investigations was of the gun-walking operation conducted by the Obama Justice Department, code named “Fast and Furious,” that was linked to a firearm used in the death of Border Patrol Agent Brian Terry in 2010.

It became a major political headache for the Obama administration and led to former Attorney General Eric Holder Eric Himpton HolderThe Hill's Campaign Report: Trump's rally risk | Biden ramps up legal team | Biden hits Trump over climate policy Biden campaign forming 'special litigation' team ahead of possible voting battle Pompeo, Engel poised for battle in contempt proceedings MORE being held in contempt by the House Oversight and Government Reform Committee for failing to share documents.

Grassley can be a partisan player.

Democrats excoriated him last year for refusing to hold confirmation hearings for President Barack Obama Barack Hussein ObamaObama calls on Senate not to fill Ginsburg's vacancy until after election Senate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week MORE’s Supreme Court nominee, Merrick Garland. It was a rare instance when Democrats accused the senior Iowa senator of extreme partisanship.

He has sided with Trump at times in criticizing the FBI. Specifically, he has pressed the FBI on its surveillance activities during the 2016 campaign, appearing to pursue the president’s allegation that he was wiretapped.

With Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-S.C.) but without his committee’s ranking member, Feinstein, he has asked the government to turn over any surveillance applications made by the FBI or the Justice Department in the course of the Russia investigation.

He also hammered former acting FBI Director Andrew McCabe — now the deputy director — for what he saw as conflicts of interest stemming from his wife’s acceptance of political donations from Virginia Gov. Terry McAuliffe (D), who is a longtime ally of Bill and Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE.

Some Republicans worry that Grassley’s investigation of Comey’s firing could give Democrats a platform to criticize the president.

Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) made clear at the end of 2016, as allegations were ramping up that Russia interfered in the presidential election, that he wanted the Senate Intelligence Committee, which is headed by his close ally Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (R-N.C.), to take the lead.

When Grassley announced his committee would take a major role as well, it left some GOP colleagues grumbling that he had been pushed into it by Feinstein or more junior GOP members of his committee.

Sen. Orrin Hatch Orrin Grant HatchBottom line Bottom line Senate GOP divided over whether they'd fill Supreme Court vacancy MORE (R-Utah), a former chairman of the panel, said he did not know of any Republicans who saw a need to investigate Comey’s firing.

“They want a political piece of the action,” said another GOP colleague who requested anonymity to comment on the motives of Judiciary Committee members.

Grassley met with Burr about six weeks ago to coordinate their efforts. Mueller’s office also gave the Judiciary panel the go-ahead to grill Manafort and Trump Jr. in a public hearing.

Stephen Kohn, a lifelong Democrat and founding partner of the nonpartisan law firm Kohn, Kohn and Colapinto, which represents federal whistleblowers, and who has worked with Grassley for 25 years, said he represents the gold standard of impartial oversight on Capitol Hill.

“I’ve been around a long time and I can tell you other senator’s and congressmen’s oversight staffs are more political,” he said. “If it’s an issue against the president they support, it’s not a good place to go. They just don’t care.”

Grassley will listen to and act on the allegations of a whistleblower whether there’s a Republican or Democratic president in office, Kohn added.

In 1984, as a junior Republican, Grassley cited Reagan Attorney General William French Smith for contempt of Congress after the Justice Department failed to turn over records about whether General Dynamics, a military contractor, submitted fraudulent cost claims to the Navy.

In an effort to block Grassley, then-Judiciary Committee Chairman Strom Thurmond (R-S.C.) refused to schedule a meeting of his subcommittee.

Grassley went around his chairman by obtaining the contempt citation through the Joint Economic Subcommittee on International Trade, Finance and Security Economics.