SPECIAL COUNSEL Robert S. Mueller III began his investigation of Russian election interference to bipartisan praise. Both Democrats and Republicans lauded the former FBI director as the ideal candidate to conduct a politically tricky investigation with rigor and honesty. Yet as Mr. Mueller's probe has continued, allies of President Trump have turned to attacking a man whose integrity and credibility they had previously considered unimpeachable. There is no basis for their criticisms.

Republicans charge that Mr. Mueller's team is hopelessly biased against Mr. Trump. Top Mueller prosecutor Andrew Weissmann, they note, attended Hillary Clinton's election-night party in November 2016. He also wrote to former acting attorney general Sally Yates after Mr. Trump dismissed her, lauding her decision not to defend the first iteration of Mr. Trump's travel ban.

Then there is Peter Strzok, an FBI agent removed from Mr. Mueller's probe after the Justice Department's Office of the Inspector General discovered that Mr. Strzok had sent text messages critical of Mr. Trump while investigating Ms. Clinton's use of a private email server. The messages exchanged by Mr. Strzok and Lisa Page, a fellow FBI employee, express crude opinions of Mr. Trump and other political figures. Mr. Strzok certainly exercised poor judgment. But the material that has so far been made public shows no sign that Mr. Strzok's work on the investigation was influenced by political bias.

Neither Mr. Weissmann's nor Mr. Strzok's activities should cast doubt on Mr. Mueller's investigation. Federal agents and prosecutors are not forbidden from holding political beliefs or giving campaign donations. The measure of their integrity is whether those beliefs infect their work, and there is no evidence that happened in either Mr. Strzok's or Mr. Weissmann's case.

Mr. Mueller's decision to remove Mr. Strzok from his team after learning of the text messages should be heartening to those concerned for the investigation's fairness. And Mr. Strzok's conduct is already the subject of an ongoing internal probe into the Justice Department's handling of the Clinton email investigation. When its work is complete, the Office of the Inspector General should release as much of its report as possible to give the public a full accounting of any misconduct. The inspector general and the Justice Department should also clarify the circumstances behind the department's unusual public release of Mr. Strzok's messages in the midst of an ongoing investigation.

Some supporters of Mr. Trump have returned to their calls to appoint a second special counsel to investigate Ms. Clinton or the Justice Department itself. But none have pointed to any factual basis for such an investigation, which would require a showing of a possible crime.

Deputy Attorney General Rod J. Rosenstein gave a vote of confidence to the special counsel's work before the House Judiciary Committee on Wednesday, affirming Mr. Mueller's integrity and clarifying that he has seen no reason to remove him from office. We are glad to see Mr. Rosenstein stand firm in support of the special counsel he appointed despite the onslaught of partisan attacks. In doing so, he is upholding no less than the rule of law.