As it turns out, Mr. Sessions met twice with Mr. Kislyak, once at the Republican National Convention in July, and again in his Senate office in September — around the time that Russian efforts to meddle in the election on behalf of Mr. Trump reached their peak. Still, meeting an ambassador is no crime in itself, which makes Mr. Sessions’s denial even more inexplicable. On Thursday, he said he “never had meetings with Russian operatives or Russian intermediaries” about the campaign. Yet a Trump administration official told CNBC’s John Harwood that Mr. Sessions had talked about the election with the ambassador, if only in “superficial” terms.

Mr. Sessions is the latest administration official to be caught between his words and the truth on Russia. Just a few weeks ago, the president fired Michael Flynn, his national security adviser, for misleading Vice President Mike Pence about his contacts with the Russian ambassador.

Mr. Sessions’s recusal is only a first necessary step. The second must be the appointment of a special counsel — an independent, nonpartisan actor who can both investigate and prosecute any criminal acts in relation to Russian interference, whether by Mr. Sessions or anyone else. That’s the only way an investigation can have credibility with the public. Simply shifting investigative authority to one of Mr. Sessions’s deputies, who report to him on all other matters, would do nothing to cure the underlying conflict.

Republican leaders in Congress also need to establish a bipartisan select committee to investigate whether the Trump campaign had a role in Russia’s election interference. Intelligence committees in both houses of Congress have said they will begin their own investigations, but those are run by the likes of Devin Nunes, chairman of the House Intelligence Committee. Mr. Nunes has dismissed concerns about the issue, and was one of several top Republicans dispatched by the White House to talk with reporters to challenge news reports tying Russia to the Trump campaign.

One person who said recusal wasn’t necessary was President Trump. Only hours before Mr. Sessions’s announcement, Mr. Trump expressed “total” confidence in his attorney general, even though he said he had not known about his communications with the ambassador. In other words, Mr. Trump appears to be saying that he has no problem with being kept in the dark.