WASHINGTON — Attorney General Jeff Sessions, facing a storm of criticism over newly disclosed contacts with the Russian ambassador to the United States, recused himself on Thursday from any investigation into charges that Russia meddled in the 2016 presidential election.

His announcement, delivered at a terse news conference, came after a day of rapid-fire developments in a murky affair that has shadowed President Trump, jeopardized his closest aides and intensified pressure for a full inquiry into Moscow’s attempts to influence the election as well as the policies of the new administration.

Many top Democrats demanded Mr. Sessions’s resignation, and a growing number of Republicans declared that he should not take part in any investigation into the case, given his own still largely unexplained role in it.

But Mr. Trump stoutly defended Mr. Sessions, one of his few early champions on Capitol Hill. “He could have stated his response more accurately, but it was clearly not intentional,” he said in a statement, which accused Democrats of engaging in “a total witch hunt.”

Mr. Sessions insisted there was nothing nefarious about his two meetings with the Russian ambassador, Sergey I. Kislyak, even though he did not disclose them to the Senate during his confirmation hearing and they occurred during the heat of the race between Hillary Clinton, the Democratic nominee, and Mr. Trump, whom Mr. Sessions was advising on national security.

In his account on Thursday of the more substantive meeting, which took place in his Senate office on Sept. 8, Mr. Sessions described Mr. Kislyak as one of a parade of envoys who seek out lawmakers like him to glean information about American policies and promote the agendas of their governments.

“Somehow, the subject of Ukraine came up,” Mr. Sessions said, recalling that the meeting grew testy after the ambassador defended Russia’s conduct toward its neighbor and heaped blame on everybody else. “I thought he was pretty much of an old-style, Soviet-type ambassador,” Mr. Sessions said, noting that he declined a lunch invitation from Mr. Kislyak.

Mr. Sessions’s decision to recuse himself was one of his first public acts as attorney general. He said he made the decision after consulting with Justice Department officials, and he denied misleading Senator Al Franken, Democrat of Minnesota, when he said in his confirmation hearing that he had not met with Russian officials about the Trump campaign.

“In retrospect,” Mr. Sessions told reporters, “I should have slowed down and said, ‘But I did meet one Russian official a couple of times, and that would be the ambassador.’ ”

The latest disclosures — and the Trump administration’s contradictory accounts of them — have deepened the questions about Russia’s role in the election and its aftermath. The affair has fueled calls for congressional and independent investigations, and toppled another close Trump aide, Michael T. Flynn, who resigned as national security adviser last month after admitting he had misled the administration over his contacts with Mr. Kislyak.

On Thursday, the White House confirmed that Mr. Flynn had his own previously undisclosed meeting with the ambassador in December to “establish a line of communication” between the incoming administration and the Russian government. Jared Kushner, Mr. Trump’s son-in-law and now a senior adviser, also participated in the meeting at Trump Tower.

The extent and frequency of the Flynn-Kislyak contacts remain unclear. But news of the meeting added to the emerging picture of how the relationship between Mr. Trump’s team and Moscow evolved to include some of Mr. Trump’s most trusted advisers.

Two other Trump campaign advisers also reportedly spoke with Mr. Kislyak last year at an event on the sidelines of the Republican National Convention.

Carter Page, a businessman and early Trump foreign policy adviser, told MSNBC on Thursday, “I’m not going to deny that I talked to him,” but said in an earlier statement that he would not comment about the event, which was off the record. Additionally, J. D. Gordon, a retired naval officer who advised Mr. Trump on national security,…