These brazen warning shots from the president do enormous damage to public confidence in the F.B.I.’s investigation. The credibility of the Justice Department in handling the Russian matter was already deeply compromised after Attorney General Jeff Sessions arrived in the job refusing to recuse himself from any investigation. He was forced to step aside only after it was revealed that, contrary to what he told senators under oath, he had met with the Russian ambassador to Washington twice during the campaign. Even with his recusal, it would still be his deputies and staff directing and managing any potential prosecution — which raises serious questions of conflict.

Mitigating this credibility crisis requires appointing an independent prosecutor, who would not take orders from the administration. If Mr. Trump’s assertion that there was no collusion between his campaign officials and the Russian government is true, he should want this matter to be fully investigated as quickly and as transparently as possible.

Appointing a special prosecutor would show that Mr. Sessions is willing to have an impartial examination of his actions as a surrogate for Mr. Trump last year — which he has assured the public were entirely appropriate.

The decision to bring in a special counsel may fall on Rod Rosenstein, a career federal prosecutor who has been nominated to be deputy attorney general. Lawmakers from both parties should strongly encourage him to make that sensible and necessary decision.

As the F.B.I. investigation continues, a series of overlapping congressional inquiries into Russian activities to influence the election are advancing in a predictably muddled, partisan way. Democrats on the House Intelligence Committee are working to produce a detailed timeline showing all the reported contacts between people close to the Trump campaign and the Russian government during the campaign. Most Republicans want to keep the focus on leaks of classified information.