WASHINGTON — House impeachment investigators on Friday planned to privately question the former United States ambassador to Ukraine, Marie L. Yovanovitch, about her knowledge of a shadow campaign by President Trump and his private lawyer to push that country’s leaders to undertake investigations that could tarnish Democrats.
The Trump administration abruptly removed Ms. Yovanovitch, a career diplomat in her third posting as an ambassador, in May, months before she was scheduled to return from Ukraine. Allies of the president had concluded that she was not sufficiently loyal to Mr. Trump, and her recall from Kiev coincided with attempts by Mr. Trump’s personal lawyer, Rudolph W. Giuliani, and his associates to jump-start an investigation into Joseph R. Biden Jr., and his son Hunter, as well as one into a debunked conspiracy theory that Ukraine meddled on behalf of Democrats in the 2016 election.
Those efforts, which extended into the State Department and the White House, where Mr. Trump pressed Ukraine’s president in July phone call to commit to the investigations, is now at the center of the House’s impeachment inquiry.
Ms. Yovanovitch could be a key witness to Mr. Giuliani’s efforts on the ground and provide investigators with insights into how Ukraine’s leaders managed the overtures, though she has given few public hints about what, if anything, she knows. Her explanation of why Mr. Trump and his allies wanted her removed could also be crucial to House Democrats who are trying to bolster their contention that Mr. Trump abused his power in pressuring Ukraine.
Ms. Yovanovitch’s expected appearance, which congressional officials said on Friday would go forward as scheduled, was itself remarkable, because she remains a Trump administration employee. The State Department blocked another high-level official from speaking with investigators on Tuesday, the same day the White House made an extraordinary declaration that it would defy the House’s every request for documents and witnesses going forward, putting a “full halt” to cooperation.
But Ms. Yovanovitch, a 33-year veteran of the State Department nearing the end of her public service, was to arrive Friday morning with a lawyer and enter the secure rooms of the House Intelligence Committee in the basement of the Capitol for questioning by congressional staff. Caught between the conflicting and equally forceful demands of two branches of government, she appears to have chosen Congress, raising the possibility that other government officials with little loyalty to Mr. Trump could follow suit.
Just a day earlier, Ms. Yovanovitch had been mentioned in an indictment of two businessmen who worked with Mr. Giuliani on his Ukraine scheme. In charging the men with federal campaign finance violations, prosecutors said they had donated funds and promised to raise more for a congressman who then lent his support to a campaign to oust her.
Three House committees conducting the investigation hope to tick through a roster of additional witness depositions next week, when lawmakers return to Washington from a two-week recess. Among them are Fiona Hill, who until this summer served as senior director for Europe at the National Security Council, is scheduled to appear Monday; George Kent, a deputy assistant secretary of state and Ukraine expert, on Tuesday; and Gordon D. Sondland, the American ambassador to the European Union whose appearance was blocked on Tuesday.
Mr. Sondland has now agreed to comply with a House subpoena and testify next week, despite the State Department’s instruction that he not appear, his lawyer said Friday.
The White House or State Department could try to block those depositions, but like Ms. Yovanovitch and Mr. Sondland, each witness may make his or her own choice.