As the flagrant anti-F.B.I. rhetoric instilled by Donald Trump and his allies seeps into the Republican mainstream, the G.O.P. is expanding its push to cast doubt on the efforts of Robert Mueller and his team. Less than two weeks after Congressman Devin Nunes was cleared of wrongdoing in a House ethics probe that sought to determine whether he disclosed classified information about the Russia investigation to the White House, the California Republican was revealed on Thursday to be the leader of a group of lawmakers working on a clandestine probe into whether Justice Department and F.B.I. officials mishandled contents of the infamous dossier compiled by ex-British spook Christopher Steele.

While it’s unclear how many are involved in the parallel probe, select Republicans on the House Intelligence Committee have been investigating the upper ranks of federal law enforcement for weeks and plan to release a report on their findings in early 2018, Politico reported. Sources familiar with the effort told the outlet that Nunes and Co. are using documents and testimony from Obama administration officials who have appeared before the intelligence panel, such as former acting deputy attorney general Sally Yates and erstwhile attorney general Loretta Lynch. (Nunes’s office, the D.O.J., and the F.B.I. declined Politico’s requests for comment.)

As Mueller has drawn closer to Trump’s inner circle, allies of the president have escalated their attacks against the special counsel. Recently, their efforts have widened to ensnare several members of the F.B.I., including Deputy Director Andrew McCabe, who Senator Chuck Grassley has said should be ousted from the bureau, and who testified for eight hours before Congress on Tuesday. Sources who spoke with Politico said the parallel inquiry led by Nunes was prompted by Republican lawmakers’ growing frustrations with the Justice Department for withholding information about both the probe and the Steele dossier. “I hate to use the word corrupt, but they’ve become at least so dirty that who’s watching the watchmen? Who’s investigating these people?” Nunes said during an interview with Fox News earlier this month, adding, “There is no one.”

Meanwhile, the Justice Department is reportedly moving to investigate the second half of the G.O.P.’s dual-pronged Mueller smear: according to NBC News, under the instruction of Attorney General Jeff Sessions, the department has begun to make inquiries into the controversial 2010 sale of a controlling stake in Uranium One—a Canadian firm that had a license to mine in the U.S.—to Rosatom, a Russian nuclear energy agency. Ultimately, the F.B.I. did not file charges in the probe, but Republicans and Trump allies have claimed that the deal, which occurred while Hillary Clinton was secretary of state, amounted to a quid pro quo. Now, prosecutors are reportedly asking F.B.I. agents to share the evidence they used in their initial investigation

Democrats have grown increasingly alarmed at the barrage of conservative attacks against Mueller and the F.B.I. “I think what we are seeing in our committee . . . is an effort to attack the Department of Justice, an effort to attack the F.B.I., to attack Mueller . . . an effort to undermine the investigations and these institutions out of fear of what they’ll find and try to discredit them in advance,” Adam Schiff, the ranking Democrat on the panel, told Politico. Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, which is also investigating Russia’s election meddling, echoed the sentiment in a speech on the floor of the upper chamber Wednesday. “The seemingly coordinated nature of these claims should alarm us all,” he said. “Let’s take a moment and remember why special counsel Mueller was appointed in the first place.”