WASHINGTON — House impeachment investigators on Wednesday summoned John R. Bolton, President Trump’s former national security adviser, and two top White House lawyers to testify next week in their inquiry into Mr. Trump’s pressure campaign on Ukraine, closing in on critical witnesses as they prepare to go public with their investigation.
Mr. Bolton, a fiery foreign policy veteran, could be a marquee player in the House’s month-old impeachment inquiry. His deputies have testified that Mr. Bolton, who left the White House in September amid disagreements with the president, was angry about the efforts to pressure Ukraine to open investigations into Democrats. Rudolph W. Giuliani, the president’s personal lawyer who was leading the charge, he warned, was a “hand grenade who’s going to blow everybody up.”
But his appearance is far from assured. His lawyer said that Mr. Bolton was “not willing to appear voluntarily,” declining to specify what his client would do should he be subpoenaed.
The much-anticipated invitation punctuated another hectic day in Washington, where talk of impeachment and the shadow diplomatic efforts at the heart of the case consumed the capital. A confirmation hearing for John Sullivan, the deputy secretary of state who is Mr. Trump’s nominee to be the ambassador to Russia, was dominated by questions about the Ukraine affair.
Under oath and on camera, Mr. Sullivan confirmed publicly for the first time that Mr. Giuliani was involved in a smear campaign to oust the ambassador to Ukraine, Marie L. Yovanovitch, and made it clear that he did not believe the president’s quest to enlist Ukraine’s political help was proper.
“I don’t think that would be in accord with our values,” he said, when asked whether it was appropriate for the president to demand investigations into domestic political opponents.
Tim Morrison, a Europe and Russia expert on the National Security Council, resigned from his post on the eve of his scheduled testimony before impeachment investigators. Mr. Morrison had been considering leaving for some time, but the timing fueled anticipation that he would offer damaging information about Mr. Trump’s conduct. NPR first reported his plans.
As the private phase of the inquiry marched forward, the House prepared for its first formal vote on Thursday related to the inquiry, as Democrats lay out rules to begin taking their impeachment process public.
And details continued to trickle out from the closed-door depositions that have been fueling the investigation. Lt. Col. Alexander S. Vindman, the top Ukraine expert at the National Security Council, told investigators on Tuesday that Ukrainian officials had complained to him about Mr. Giuliani and his demands for investigations. He went so far as to warn Volodymyr Zelensky as he was being sworn in as the president of Ukraine, according to people familiar with his testimony. During a May meeting in Ukraine to mark Mr. Zelensky’s inauguration, the colonel advised him to try to avoid becoming ensnared in politics in the United States.
In addition to Mr. Bolton, House investigators sent requests on Wednesday to two senior White House lawyers implicated in the case, John A. Eisenberg and Michael Ellis, who could offer important insight into the events under scrutiny. Mr. Eisenberg fielded concerns from Mr. Bolton’s deputies about apparent demands being placed on Ukraine and helped move a transcript of a now-famous July phone call between Mr. Trump and Ukraine’s leader into a secure server that limited who could see it.
“Based upon public reporting and evidence gathered as part of the impeachment inquiry, we believe that you have information relevant to these matters,” three Democratic committee leaders wrote in a short letters to all three men. The letters did not include subpoenas.
Many Democrats regard Mr. Bolton as the perfect witness, a respected conservative national security hawk who was nonetheless incensed by the how the president and his inner circle were treating Ukraine, and who broke sharply with the president upon his departure from the White House.
If Mr. Bolton is subpoenaed, his lawyer, Charles J. Cooper, is likely to ask a federal judge to determine whether his client needs to comply.
Mr. Cooper, who also represents Mr. Bolton’s former White House deputy, advised the deputy, Charles M. Kupperman, not to comply with a subpoena to appear this week after the White House intervened. Instead, he filed a lawsuit asking a federal judge whether he should testify in the impeachment inquiry. A hearing on that matter is scheduled for Thursday at the United States District Court in Washington. Mr. Bolton could take a similar course.
Mr. Eisenberg and Mr. Ellis could face similar decisions about whether to satisfy the White House directives or congressional demands. As White House lawyers, they could also be subject to certain special privilege claims to shield their testimony from Congress.
On Wednesday, Democratic and Republican investigators privately questioned two more witnesses, both foreign service officers who worked closely on Ukraine policy at the White House, advising Kurt D. Volker, the former United States special envoy to that country.
They offered additional information about concerns about Mr. Giuliani’s role among some senior administration officials, including Mr. Bolton, and gave intriguing new details about a campaign to smear Ms. Yovanovitch.
One of them testified that Robert Livingston, a former Republican congressman turned lobbyist, repeatedly told her when she was assigned to the White House that the ambassador should be fired because of her association with Democrats.
The officer, Catherine M. Croft, testified that she “documented” multiple calls from Mr. Livingston about Ms. Yovanovitch while she was working at the National Security Council from mid-2017 to mid-2018. She said that she informed two other officials — Fiona Hill, then the senior director for Europe and Russia at the council, and George P. Kent, a Ukraine expert at the State Department — about them at the time.
“He characterized Ambassador Yovanovitch as an ‘Obama holdover’ and associated with George Soros,” she said, referring to the billionaire liberal philanthropist, according to a copy of Ms. Croft’s opening statement reviewed by The New York Times. “It was not clear to me at the time — or now — at whose direction or at whose expense Mr. Livingston was seeking the removal of Ambassador Yovanovitch.”
Mr. Livingston and his firm did not reply to requests for comment on Wednesday.
Investigators are scrutinizing smears against Ms. Yovanovitch to understand if they were part of a larger pressure campaign by Mr. Trump and Mr. Giuliani to secure from Ukraine politically beneficial investigations into Democrats. Mr. Trump eventually recalled her this spring from Kiev, months ahead of schedule, but the ambassador was told he had been intent on removing her since the summer of 2018.
The on-the-record statements about the campaign by Mr. Sullivan were significant, and they drew an angry response on Twitter from Mr. Giuliani, who described his testimony as an “orchestrated attempt to harass and hinder me in my role as” Mr. Trump’s lawyer.
“The Amb. nominee doesn’t know what he’s talking about and shouldn’t be incorrectly speculating,” he wrote.
In the House, investigators also pressed Christopher J. Anderson, who preceded Ms. Croft as Mr. Volker’s adviser, on aspects of testimony given earlier by Mr. Volker and fill in details about his work trying to manage the demands of Mr. Trump and Mr. Giuliani on Mr. Zelensky.
Mr. Volker told investigators that he had not been aware of any quid pro quo demanded by Mr. Trump, but he detailed how Mr. Giuliani pressed the Ukrainians to publicly pledge that they would undertake investigations that could damage the president’s domestic political adversaries. And text messages he shared with Congress at least appeared to show that a coveted White House meeting for Mr. Zelensky would come only if they agreed to certain investigations.
According to a copy of his opening statement, Mr. Anderson testified that he and Mr. Volker worked to appease Mr. Giuliani as they tried to help Ukraine’s new government root out corruption in general and deepen its ties to the United States — but bumped up against him again and again.
Mr. Anderson described a June 13 meeting at the White House with Mr. Volker and Mr. Bolton, in which Mr. Bolton indicated that Mr. Giuliani could pose a problem as they sought to build more support for Mr. Zelensky among senior White House officials.
“He cautioned that Mr. Giuliani was a key voice with the president on Ukraine, which could be an obstacle to increased White House engagement,” Mr. Anderson said. He added that he wrote a summary of Mr. Bolton’s remarks about Mr. Giuliani and shared it with Mr. Kent and others at the State Department.
During another meeting of senior officials at the Energy Department a few days later, Mr. Anderson said, there were “vague discussions in the meeting about how to address Mr. Giuliani’s continued calls for a corruption investigation.”
Reporting was contributed by Danny Hakim, Catie Edmondson, Julian E. Barnes, Sharon LaFraniere and Michael S. Schmidt.