WASHINGTON — The White House’s trenchant declaration to House impeachment investigators last week was unequivocal: No more witnesses or documents for a “totally compromised kangaroo court.”
But just a week later, it has become clear that President Trump’s attempts to stonewall the Democrat-led inquiry that has imperiled his presidency and ensnared much of his inner circle are crumbling.
One by one, a parade of Trump administration career diplomats and senior officials has offered a cascade of revelations. Those accounts have corroborated and expanded upon key aspects of the whistle-blower complaint that spawned the impeachment inquiry into whether the president abused his power to enlist Ukraine to help him in the 2020 presidential election.
The latest disclosures came on Wednesday, when a former top aide to Secretary of State Mike Pompeo offered an inside account of what he said was a demoralized State Department, where career diplomats were sidelined and others apparently were pressed to use their posts “to advance domestic political objectives.” In six hours of voluntary testimony, the former aide, Michael McKinley, told impeachment investigators that he quit his post as Mr. Pompeo’s senior adviser amid mounting frustrations over the Trump administration’s treatment of diplomats and its failure to support them in the face of the impeachment inquiry, according to a copy of his opening remarks.
On Thursday, Democrats are set to hear from Gordon D. Sondland, the United States ambassador to the European Union, a central figure in the president’s pressure campaign on Ukraine. He is expected to testify that he learned that Mr. Trump did not intend to invite President Volodymyr Zelensky of Ukraine to a meeting in the Oval Office until Mr. Zelensky pledged to open an investigation that could benefit Mr. Trump’s political fortunes — bolstering a central allegation in the inquiry that the president steered foreign policy for political gain.
And Democratic lawmakers have directed William B. Taylor Jr., one of the top American diplomats in Ukraine, to appear before their committees next Tuesday, according to an official familiar with the investigation. Text messages produced as part of the inquiry suggest that Mr. Taylor was deeply uneasy about what he saw as an effort by Trump aides to use a $391 million package of security assistance as leverage over Ukraine for political favors, calling the notion “crazy.”
All three are examples of what can happen when Congress secures cooperation from government witnesses in a rapidly moving investigation aimed at the president.
The White House has had more success blocking the release of documents tied to the case. But the president and his lawyers had hoped to use the power of his office to muzzle current and former diplomats and White House aides, arguing in presidential tweets and a lengthy letter to Democratic lawmakers on Oct. 8 that their subpoenas are invalid and unenforceable.
“President Trump cannot permit his administration to participate in this partisan inquiry under these circumstances,” wrote Pat Cipollone, the White House counsel.
And yet the president has been unable to prevent it.
Just since Mr. Trump declared war on the impeachment effort, three current and former senior State Department officials and a former top White House aide have testified for nearly 36 total hours, delivering to lawmakers a consistent narrative of how they were effectively pushed aside by allies of the president operating outside America’s usual foreign policy channels.
“It’s partly because this shadow foreign policy that the president was running was so deeply offensive to people in his own administration who took pride in overseeing a professionally run and arguably exemplary policy in support of Ukraine,” said Representative Tom Malinowski, Democrat of New Jersey and a former State Department official involved in the inquiry. Referring to Mr. Trump’s personal lawyer, he added, “And then to see the official policy undermined by this clownishly corrupt effort led by Rudy Giuliani on behalf of the president was just more than many people apparently could bear.”
Republicans who control the Senate view the fast-building case as serious enough to begin preparing for the trial in their chamber that would follow impeachment by the House. Senator Mitch McConnell, Republican of Kentucky and the majority leader, briefed fellow lawmakers over lunch on Wednesday about how a trial would work, expressing his hope of conducting it speedily and completing it by the end of the year, people familiar with his remarks said.
Facing accusations of secrecy from Republicans, Representative Adam B. Schiff, Democrat of California and the chairman of the Intelligence Committee, informed colleagues on Wednesday that he planned to open the inquiry to the public soon. He wrote that he planned to release transcripts of all the interviews as the investigation proceeded and pledged to soon hold public hearings “so that the full Congress and the American people can hear their testimony firsthand.”
For Mr. Trump, who is famous for demanding fierce loyalty from those around him, the daily — or even hourly — crush of damaging headlines is an infuriating departure from previous successes in controlling disclosures to Congress from people in his orbit.
During the congressional investigation into Russia’s election meddling, Mr. Trump blocked a deposition of Donald F. McGahn II, his former White House counsel, and dramatically limited testimony from some of his closest aides, including Hope Hicks, the former White House communications director, and Corey Lewandowski, his former campaign manager.
But this is different. Many administration officials targeted for depositions by Democrats are diplomatic veterans who have expressed anger and frustration about what they described as the hijacking of American foreign policy. They have no particular loyalty to Mr. Trump, nor are they subject to the same presidential powers to block them from testifying.
So they have turned up at the secure suite of the House Intelligence Committee on Capitol Hill, disappearing behind doors with a red “RESTRICTED AREA” sign to tell their stories.
Under alternating hourlong question-and-answer sessions by Democratic and Republican staff lawyers, Marie L. Yovanovitch, the former ambassador to Ukraine, said she had been ousted at Mr. Trump’s direction on the basis of “unfounded and false claims.” Fiona Hill, a former National Security Council aide, said John R. Bolton, then the national security adviser, was so alarmed by the activities of Mr. Giuliani, Mr. Sondland and others that he instructed her to alert White House lawyers. She said she reported Mr. Sondland to intelligence officials as a possible national security risk as well.
The decision by Ms. Yovanovitch, Ms. Hill and others to testify is a demonstration of the limits of presidential power and the legal constraints Mr. Trump is under as he and his lawyers try to devise a strategy for keeping him in office.
Although the White House has struggled to keep former officials from agreeing to testify, Mr. Trump has more leverage with current administration employees, who may fear for their jobs if they defy the blockade. But it is not clear what the political repercussions would be if the president retaliated against them in the middle of a political scandal.
Mr. McKinley told investigators on Wednesday that State Department officials were discouraging people from testifying, and were not supporting diplomats who had received subpoenas and requests to appear before the House, according to a person familiar with his testimony.
Mr. Trump’s allies on Capitol Hill expressed frustration this week about the depositions, saying White House lawyers should be present and accusing Democrats of selectively leaking from the testimony. Others were simply baffled by the cooperation of the witnesses.
“I really don’t understand it,” said Representative Chris Stewart, Republican of Utah and a member of the Intelligence Committee. “I can’t wrap my head around why some and why not others.”
Veterans of past legal struggles between the White House and Congress said Mr. Trump was confronting the reality that he had limited ability to force former or even current government employees to ignore a legally binding subpoena. It is even difficult — though not impossible — to shield top White House aides from appearing, they said.
“Particularly if there’s a subpoena, everybody has to appear or risk being held in contempt,” said W. Neil Eggleston, who served as President Barack Obama’s White House counsel. “It is just not easy to simply refuse to appear.”
Mr. Eggleston said that defying a subpoena was sometimes possible for high-profile figures, but was especially difficult for functionaries and other career employees.
Mr. Trump’s lawyers have had more success in blocking access to emails, text messages, memos and other documents in the government’s possession.
The administration has rejected Democratic subpoenas or requests for documents at the Office of Management and Budget, the State Department, the Defense Department and the office of Vice President Mike Pence. Mr. Giuliani has also said he will ignore a subpoena for his records, citing the White House’s stance.
Democrats have said the refusal to hand over documents will be considered obstruction of Congress and may be added to the impeachment charges brought against the president.
The White House has also attempted to limit the questions witnesses can answer.
In the case of Ms. Hill, White House lawyers conceded early Monday that they could not stop her from arriving on Capitol Hill for a deposition by the committee later that day, but they demanded that she refrain from speaking about classified material, conversations with the president and other matters.
Even that proved difficult to enforce, as Ms. Hill vividly described a dramatic confrontation inside the White House between Mr. Bolton and Mr. Sondland.
Mr. Schiff, the Intelligence Committee chairman, said Tuesday that the sessions with witnesses have been fruitful despite the efforts to block them.
“It’s a way of trying to chill them from cooperating,” Mr. Schiff said. “It’s not working, but I think that’s the goal.”
“It goes to show the legally insupportable position of the White House,” he added.
Julian E. Barnes, Sharon LaFraniere and Michael S. Schmidt contributed reporting.