WASHINGTON — The continuing fallout from President Trump’s unsubstantiated wiretapping allegation cost him another ally on Thursday, as the embattled Republican chairman of the House Intelligence Committee announced he would step aside from his panel’s investigation into Russia’s efforts to disrupt last year’s election.

The announcement from the committee’s chairman, Representative Devin Nunes of California, came shortly before the House Committee on Ethics said he was under investigation because of public reports that he “may have made unauthorized disclosures of classified information.”

Mr. Nunes’s recusal from the Russia inquiry was a blow to Mr. Trump, who in less than three months in office has seen the imbroglio over Russia’s disruption campaign exact political damage on some of his closest advisers and most vigorous supporters. The furor over the contacts that some of the president’s aides had with Russian officials has already led to the firing of Michael T. Flynn, Mr. Trump’s national security adviser, and the recusal of Attorney General Jeff Sessions from overseeing the F.B.I.’s inquiry into the Russian efforts.

Now it has led to the recusal of Mr. Nunes from the House Intelligence Committee’s Russia investigation, and Mr. Trump faces the potential revival of that inquiry. The investigation had descended into a partisan sideshow in the two weeks since Mr. Nunes told reporters that Trump associates had been swept up in surveillance of foreign officials by American spy agencies during the transition, citing intelligence reports that were classified.