Tom Fitton, president of the conservative watchdog group Judicial Watch, said “our concerns about Mueller are beginning to take hold.” (Linda Davidson/The Washington Post)

For months, efforts to discredit special counsel Robert S. Mueller III’s investigation into Russian meddling in the 2016 campaign flickered at the fringes of political debate.

Now, the allegation that FBI and Justice Department officials are part of a broad conspiracy against President Trump is suddenly center stage, amplified by conservative activists, GOP lawmakers, right-leaning media and the president himself. The clamor has become a sustained backdrop to the special counsel investigation, with congressional committees grilling a parade of law enforcement officials in recent days.

“Until recently, it has been a lonely battle,” said Tom Fitton, whose conservative watchdog group Judicial Watch has helped drive the charges by unearthing internal Justice Department documents. “Our concerns about Mueller are beginning to take hold.”

The partisan atmosphere is a sharp departure from the near-universal support that greeted Mueller's selection as special counsel in May — and threatens to shadow his investigation's eventual findings. Trump, while vowing to cooperate with the special counsel, has also encouraged attacks on Mueller's credibility, tweeting that the investigation is "the greatest Witch Hunt in U.S. political history."

The controversy, percolating for months, escalated dramatically in early December with the revelation of text messages in which one of Mueller’s former top investigators, Peter Strzok, called Trump an “idiot” last year and predicted Democratic candidate Hillary Clinton would win the election in a landslide.

Special counsel Robert S. Mueller departs the U.S. Capitol after a closed-door meeting with members of the Senate Judiciary Committee in June. (J. Scott Applewhite/AP)

As the deputy head of counterintelligence at the FBI, Strzok played a critical role in both the Clinton email investigation last year and the Russia probe before he was removed by Mueller this summer.

Rep. Jim Jordan (R-Ohio), who met with Fitton earlier this year and has for months alleged that the FBI was working against Trump’s election, said in an interview that many of his Republican colleagues now share his view that there has been an orchestrated effort against Trump.

“I’ve had all kinds of Republicans come up to me and say, ‘This is unbelievable, it looks like the FBI was trying to put its finger on the scale here,’ ” Jordan said.

Among current and former law enforcement officials, the public attacks on the FBI are seen as an indirect way of trying to discredit Mueller and blunt future findings he may issue, a view shared by many Democratic lawmakers.

“There is a concerted push from the White House . . . and their allies to bring the investigations to a halt,” Rep. Adam B. Schiff, the top Democrat on the House Intelligence Committee, said in an interview. “They are also trying to attack Mueller’s credibility and the credibility of the FBI, so that whatever Mueller finds can be rejected . . . as a fake.

“The White House would like to have the best of both worlds,” he added. “They make the public case that they are cooperating, while their allies do the dirty work.”

In response, Ty Cobb, the White House lawyer overseeing the response to the Russia investigation, said in a statement that “the President respects the Special Counsel and his process and will continue to fully cooperate with the Special Counsel.”

A spokesman for Mueller declined to comment.

Some of the key players in the campaign against the special counsel probe are veterans of politically charged investigations, having helped drive attacks against the Clintons in the 1990s and during last year’s presidential campaign.

One leading critic is David Bossie, a former Trump deputy campaign manager. He was a congressional investigator who examined President Bill Clinton’s campaign finances in the late 1990s and currently leads Citizens United, a conservative advocacy group that produced movies critical of Hillary Clinton and other Democrats.

Bossie now makes frequent appearances on Fox News and other conservative media outlets, arguing that the special counsel is being used to try to delegitimize Trump. He said it is crucial to make a sustained fight against the probe.

“It is not that I wake up and say, ‘How do I match the Clinton playbook?’ ” Bossie said. “I just have the experience of understanding the rapid-response aspect of messaging. You have to be out there with a counter, set-the-facts-straight message or highlight what the problems are very quickly, or these things get away from you.”

He argues there is no evidence that the Trump campaign colluded with Russia.

“I’m not against Mueller; I’m against the concept of an investigation as a red herring,” he said.

Fitton’s Judicial Watch group, too, has a long history of investigating the Clintons, having filed numerous lawsuits against the administration of President Bill Clinton. During the 2016 campaign, the organization obtained thousands of emails written by Hillary Clinton when she was secretary of state.

This year, Judicial Watch has helped stoke the attacks against the Mueller probe with material it obtained through lawsuits and Freedom of Information Act requests. The nonprofit group, which has a $35 million budget and 50 employees, does not release the names of its approximately 500,000 donors, Fitton said.

Fitton has frequently gone on Fox News, conservative websites and Twitter to report his findings. On Dec. 2, after Fitton tweeted that Trump "needs to clean house at FBI/DOJ," Trump retweeted another user's summary of Fitton's statement.

In one email obtained through a Judicial Watch lawsuit, Andrew Weissmann, a senior lawyer working for Mueller, wrote in January that he was “so proud” of then-acting attorney general Sally Yates’s decision to defy Trump’s executive order banning travel by certain immigrants. The FOIA request was filed in May and was received in the fall, Fitton said. Other requests have taken longer or been rejected altogether, he said.

Judicial Watch also obtained emails regarding FBI Deputy Director Andrew McCabe that the group says show he was involved in helping his wife, Jill, run as a Democratic candidate for a state Senate seat in Virginia.

McCabe was told in one email that then-Director James B. Comey had "no issue" with McCabe's wife seeking the seat. Another document said Hillary Clinton attended a June 2015 fundraiser for Virginia Gov. Terry McAuliffe's political action committee, which in turn gave nearly $500,000 to Jill McCabe for her state Senate bid.

Republicans have also raised questions about the FBI’s handling of a dossier produced by Christopher Steele, a former British spy who was hired by a research firm called Fusion GPS to investigate Trump’s ties to Russia. Senate Intelligence Committee investigators on Thursday interviewed Bruce Ohr, a Justice Department official whose wife, Nellie, worked for Fusion GPS in 2016.

In a recent court filing, Fusion GPS said it was being targeted by congressional committees “coordinating with the President [and] his personal lawyers . . . to misdirect attention to Fusion . . . due to their perceived role in exposing the ties between the Trump campaign and the Russians.”

Jay Sekulow, one of Trump’s outside attorneys, has called for a second special counsel to be appointed to investigate the Fusion GPS matter. He said in an interview that his proposal “is in no way related to Robert Mueller,” with whom he said he has “a professional and cooperative relationship.”

The pressure on Mueller’s team has increased as prosecutors unveiled charges this fall against four former Trump advisers.

Less than two weeks after former national security adviser Michael Flynn pleaded guilty to lying to the FBI, the Justice Department disclosed anti-Trump and pro-Clinton texts that Strzok exchanged with another senior FBI official, Lisa Page, while they were having an affair and managing sensitive political investigations of those candidates.

The texts were uncovered in July by the Justice Department’s inspector general, which has been investigating FBI decision-making during the probe into Hillary Clinton’s use of a private email server when she was secretary of state.

When the inspector general warned Mueller in the summer about what the probe had found, Mueller immediately removed Strzok from his team. Strzok was reassigned to a job in the FBI's human resources division. Page had also worked on the Mueller team but left two weeks earlier for what officials said were unrelated reasons.

The Post has reported that in a 2015 text, Strzok said that Sen. Bernie Sanders (Vt.), who sought the Democratic nomination against Clinton, was "an idiot like Trump. Figure they cancel each other out." In 2016 texts, Strzok wrote, "God, Hillary should win 100,000,000-0" and that he was "worried about what happens if [Clinton] is elected."

Former colleagues defended him, saying Strzok’s opinions had no impact on how he conducted investigations.

“To think Pete could not do his job objectively shows no understanding of the organization,” said Michael Steinbach, former executive assistant director of the FBI’s National Security Branch, adding: “We have Democrats, we have Republicans, we have conservatives and liberals. . . . Having personal views doesn’t prevent us from independently following the facts.”

But as news of Strzok's text messages spread, Trump jumped on the story, tweeting: "Report: 'ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE' Now it all starts to make sense!"

Republicans in Congress took the cue, seizing upon the texts to attack the credibility of the FBI and the Mueller investigation.

“The senior levels of the FBI have been infected with an intractable bias that seemed to favor Hillary Clinton and work against President Donald Trump,” said Rep. Matt Gaetz of Florida on Fox News on Wednesday, adding, “It’s time for Bob Mueller to put up or shut up: If there’s evidence of collusion, let’s see it.”

The calls for Mueller’s ouster are strongest in the House, where a group of Republicans has been calling for the special counsel to resign.

House Speaker Paul D. Ryan (R-Wis.) has said Mueller’s investigation should proceed without interference. But he has allowed several committee investigations that are calling into question the integrity of the probe.

“The House has a constitutional obligation to exercise congressional oversight, and the speaker is supportive of our committee chairmen carrying out their work,” said Ryan spokeswoman AshLee Strong.

In recent days, for example, three House committees grilled McCabe over his participation in the FBI’s Russia investigation and his role in the FBI examination into Clinton’s use of a private email server.

Democrats called it a thinly veiled attempt to weaken McCabe and slow down Mueller's probe. McCabe plans to retire in a few months when he becomes fully eligible for pension benefits, people familiar with the matter told The Post.

“Those people should be investigating the real crime, which is Russia’s interference in our democracy, and instead they’re being hauled before a six-hour series of interviews,” said Rep. Raja Krishnamoorthi (D-Ill.).

At the House Intelligence Committee, Rep. Devin Nunes (R-Calif.) and a small group of Republican lawmakers are discussing writing a report next year that would highlight alleged “corruption” at the FBI, according to people familiar with the plans. Such a report would focus on information about the conduct of FBI officials in the course of the Russia investigation, those people said.

On the Senate side, one of the loudest voices has been Republican Charles E. Grassley of Iowa, who chairs the Judiciary Committee and has raised questions about the impartiality of Mueller’s probe.

He has called for McCabe to be fired and shown a willingness to dig into Mueller's past tenure as FBI director, complaining Thursday that the FBI and the Justice Department have been too slow to rout out people peddling "political influence."

Grassley has also called for a second special counsel to look at decisions the FBI and the Justice Department made at the time that the Obama administration approved a uranium deal giving Russia a significant stake in the U.S. market. The inquiry would bring de facto scrutiny of Mueller, who was FBI director at the time.

Grassley said that his staff is in touch with Nunes’s staff, though he would not specify exactly what elements of their committees’ parallel inquiries they were communicating about.

“I wouldn’t want to say there’s coordination,” Grassley said. “There’s communication.”

He insisted that he was not aiming to discredit Mueller, adding that he has “got confidence [Mueller’s] going to be able to do what he’s doing.”

Trump, meanwhile, has continued to urge on the questions. This month, he tweeted that after the FBI's "phony and dishonest Clinton investigation (and more) . . . its reputation is in Tatters - worst in History! But fear not, we will bring it back to greatness."