Several law enforcement officials said they are concerned that the constant drumbeat of conservative criticism seems designed to erode Mueller's credibility, making it more politically palatable to remove, restrict or simply ignore his recommendations as his investigation progresses.
Fox News Channel personality Sean Hannity, one of the president's informal advisers as well as one of his most vociferous defenders, on Tuesday night called Mueller "a disgrace to the American justice system'' and said his team is "corrupt, abusively biased and political.''
Several conservative lawmakers held a news conference Wednesday demanding more details of how the FBI proceeded last year in its probes of Hillary Clinton's use of personal email and Russian election interference. This week, the conservative group Judicial Watch released an internal Justice Department email that, the group said, showed political bias against Trump by one of Mueller's senior prosecutors.
Fresh ammunition came this weekend, when it was revealed that Peter Strzok, the top FBI agent on Mueller's team, had been removed over politically charged texts he'd exchanged with another former member of the Mueller team, senior FBI lawyer Lisa Page. The texts appeared to favor Clinton and disparage Trump, according to people familiar with the matter.
"The question really is, if Mueller was doing such a great job on investigating the Russian collusion, why could he have not found the conflict of interest within their own agency?'' Rep. Mark Meadows (R-N.C.) asked at the news conference. Meadows, leader of the Freedom Caucus, cited a litany of other issues that he said show bias on the part of the FBI and Mueller, including past political donations by lawyers on Mueller's team.
An FBI spokesman declined to comment.
Accusations of bias against Mueller from conservatives have become commonplace in the public debate about the president and the Russia probe, and Republicans are expected to grill FBI Director Christopher A. Wray about those matters when he testifies Thursday before the House Judiciary Committee.
The chairman of that committee has been pressing the Justice Department to appoint a second special counsel — one to probe Clinton, as well as the FBI's handling of past Clinton-related probes. Law enforcement officials also expect Wray will be pressed on that issue again Thursday in the wake of the Strzok-Page revelations, which are being investigated by the Justice Department's inspector general.
Mueller did get a public vote of confidence Wednesday from Deputy Attorney General Rod J. Rosenstein, the senior Justice Department official overseeing the Russia probe — though Rosenstein did not address the Strzok inquiry. In an interview with NBC, Rosenstein was asked whether he was satisfied with what he had seen so far from the special counsel's office, and he said yes and noted that some public charges had been filed. "We're not in a position to talk about anything else that may be going on,'' he said.
Mueller first became aware in late July of text messages exchanged between Page and Strzok, who had been engaged in an affair, according to people familiar with the matter who, like others, spoke on the condition of anonymity to discuss internal deliberations.
Strzok was removed from the job immediately and transferred to the FBI's human resources division, which was widely understood by his colleagues to be a demotion. Officials have said Page left the Mueller team two weeks earlier for unrelated reasons.
Trump tweeted this weekend that the FBI's reputation was "in Tatters.''
Strzok was a major player in both the Clinton and Russia probes, taking part in key interviews, including those of Clinton and Trump's former national security adviser, Michael Flynn, who pleaded guilty last week to lying to the FBI during that January questioning.
On Wednesday, Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) signed letters to the Justice Department and FBI demanding more information about Strzok's communications.
"Strzok's behavior and involvement in these two politically-sensitive cases raises new concerns of inappropriate political influence in the work of the FBI,'' Grassley wrote in one of the letters.
Matthew Miller, a Democrat and former Justice Department spokesman, said Grassley is part of a Republican effort to undermine Mueller's credibility over the long run.
"First, they want to kick up dust about Hillary Clinton so the conservative press has something to talk about that isn't Trump's misdeeds,'' Miller said. "The eventual goal, though, is to delegitimize Mueller in such a way that he can either be fired or can be ignored if he concludes the president broke the law.''
A Grassley spokesman called Miller's comment "a baseless charge from a Democratic operative'' and said the senator has a "three-decade record of government oversight across administrations.''
Grassley also called Mueller an "honorable person" whose investigation should be allowed to "play out."
Tom Fitton, the president of Judicial Watch, has called for the Mueller probe to be shut down, saying his prosecutors are simply too biased against the president to conduct a credible investigation.
Fitton said the Justice Department and FBI "covered up'' the Strzok issue for months. "That's a scandal," he said. "Rosenstein needs to explain what he was doing, what he knew and when, and Mueller needs to explain himself as well. I think Mueller has fewer and fewer supporters in the Republican establishment, because of what he allowed to happen.''
The email released by Judicial Watch this week was sent by Andrew Weissmann, now on the Mueller team, back in January, when he was a senior Justice Department official in the criminal division. After then-acting attorney general Sally Yates was fired for instructing department employees not to defend Trump's first travel ban in court, Weissmann sent her a note saying he was "so proud and in awe'' of her. Judicial Watch said the email shows Weissmann is biased against the president.
In Congress, an effort by a Republican lawmaker to ensure Mueller could not be abruptly fired has lost steam.
Sen. Thom Tillis (R-N.C.), who in August unveiled legislation to prevent Trump from firing Mueller without cause, said Wednesday that he felt no urgency for the Senate to take it up.
"Based on what's occurred with Flynn and some of the reports over the past week, I'm not overly concerned that we have to move quickly," Tillis said. He called his bill a "good governance" measure that lawmakers will continue to discuss.
Tillis offered a mixed review of the Mueller probe.
"Some of the questions raised about some of the people in the FBI and their behavior and possible biases make you want to go back and look at the role that they played and whether or not there was any bias that was woven into any results or observations they made," Tillis said. "But on the whole, I'm satisfied with the way it's progressing."
Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.), one of his party's most outspoken Trump critics, said he couldn't envision the president firing Mueller.
"I can't imagine him being terminated," Corker said. "To me, that would be a step too far."
As for the way the Mueller investigation is proceeding, Corker declined to opine. "I have almost no knowledge as to how it's proceeding," he said.