ONCE AGAIN, President Trump has launched a round of attacks on federal law- ­enforcement agencies. Mr. Trump's fury at the FBI and the Justice Department is now familiar, as is its purpose: to discredit special counsel Robert S. Mueller III's investigation into Russian election interference and degrade the independence of law enforcement. The only novelty lies in the question of what or who he will vilify next.

The most recent subjects of the president's frustration include two top career FBI officials, Deputy Director Andrew McCabe and former general counsel James Baker. Without evidence, Mr. Trump hinted darkly on Twitter at some wrongdoing by Mr. Baker in noting his reassignment from the general counsel position. Mr. Trump reiterated his attacks from this summer on Mr. McCabe, accusing the deputy director of political bias because of donations by Virginia Gov. Terry McAuliffe (D) to a failed state senate campaign run by Mr. McCabe's wife. Yet there is no law prohibiting the spouses of civil servants from running for political office. And Mr. McCabe did no work for the campaign, which had ended by the time he was assigned to the Hillary Clinton email investigation.

This has not stopped congressional Republicans from calling for the head of the deputy director, who plans to retire in March. Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) declared that Mr. McCabe should depart the bureau to ensure that "there's not undue political influence within the FBI." Likewise, Republican members of the House Intelligence Committee have baselessly suggested that Mr. Baker was somehow involved in providing to the media information about a private investigator's dossier on Mr. Trump's alleged Russian connections.

The criticism of Mr. McCabe and Mr. Baker is just one more attempt to kick up dust around the special counsel's investigation, along with overblown concerns regarding the credibility of Mr. Mueller's team and scandalmongering over the sale of uranium mining rights to a Russian company during Ms. Clinton's tenure as secretary of state. Despite Mr. Trump's belief that the probe is "bad for the country," the White House insists that the president has no intention of firing Mr. Mueller. But with the help of his allies in Congress and the media, Mr. Trump has been laying the groundwork to discredit the special counsel's eventual findings by painting the FBI as dishonest and corrupt.

In assisting the president's efforts to erode public trust in Mr. Mueller, congressional Republicans are also abetting his attacks on the independence and integrity of federal law enforcement as a whole. "I have absolute right to do what I want to do with the Justice Department," Mr. Trump claimed this last week. The foundations of our democracy rest on the opposite being true: that no one is above the law and that justice will be dealt out evenhandedly, not on the basis of political vendettas.

Calls for a partisan housecleaning of federal law enforcement will only make it easier for Mr. Trump to turn the FBI and the Justice Department into the politicized shells he already imagines them to be.