A former U.S. attorney general under Presidents Reagan and George H.W. Bush is calling on members of Congress — Republicans included — to safeguard the special counsel investigation into Russia's role in the 2016 election.

In an op-ed published by The Washington Post on Saturday, Dick Thornburgh expressed concern about what he called "unfair attacks on the Justice Department" perpetrated by some lawmakers and the White House.

He also voiced confidence for special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE, who is carrying out the criminal investigation into possible collusion between the Trump campaign and Russia, praising him as "serious but not sensational and loyal to the rule of law."

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"Elected officials must stress unambiguously their support of the role of the special counsel and guard his right to pursue this investigation in a fair and impartial manner," Thornburgh wrote.

"Through whatever means members of Congress deem appropriate, they must defend the rule of law and the integrity of the special counsel’s mission."

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE himself has repeatedly attacked the FBI, the Justice Department and the special counsel investigation, which he deems a "witch hunt" and a "hoax."

In recent weeks, he and some of his allies have floated the notion that the FBI planted a spy in his 2016 presidential campaign, though no evidence has surfaced to back up that claim. Trump also asserted twice this week that he has the power to pardon himself, should he face criminal charges, but insisted that he has done nothing wrong.

At the same time, some congressional Republicans have questioned the special counsel investigation, as well as the FBI and Justice Department's actions during the 2016 presidential race.

Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) this week waved off Trump and his allies' claims that the FBI had spied on his campaign, and also asserted that the president cannot pardon himself.

"Nobody is above the law," he said.