WASHINGTON — House impeachment investigators met for a rare weekend session on Saturday to privately question a senior official from the White House budget office about President Trump’s decision this summer to freeze $391 million in security assistance to Ukraine.
Why precisely Mr. Trump withheld the congressionally allocated funding in mid-July as he pressed Ukraine for politically beneficial investigations and what his acting chief of staff, Mick Mulvaney, told the agency about the decision remain central unanswered questions in the inquiry.
Democrats leading the proceedings hope that the budget official, Mark Sandy, can at least offer a glimpse into deliberations at the Office of Management and Budget over carrying out the order.
Other witnesses with significant roles in American diplomacy toward Ukraine, including some working closely with Mr. Trump, have already testified that the aid was delayed as part of a broad pressure campaign meant to extract a public commitment from Ukraine to investigate the president’s political rivals.
Democrats have marshaled that testimony to begin arguing that Mr. Trump may have committed bribery to get what he wanted from Ukraine. But they have yet to hear from a witness who can speak directly to the president’s order and stated rationale.
Mr. Sandy testified after the House Intelligence Committee subpoenaed him Saturday morning, a day after the former American ambassador to Ukraine, Marie L. Yovanovitch, described in stark and personal terms how the president and his allies sought to undermine her and push her out of her job. The budget office had directed him not to appear, according to an official working on the inquiry.
Mr. Trump continued his scorched-earth defense on Saturday, denouncing those involved in the proceedings. In one tweet, he misspelled the name of the chairman of the House Intelligence Committee leading the inquiry, Representative Adam B. Schiff of California, to what sounds like a vulgarity, claiming the stock markets would collapse if he were impeached. And the president, attributing a quotation to the conservative radio host Rush Limbaugh, suggested that nonpartisan diplomats who have testified were aggrieved members of the Washington “Swamp” merely trying to exact their revenge.
“It is paramountly obvious watching this, these people have to go,” Mr. Limbaugh said, according to the president. On Friday, Mr. Trump targeted Ms. Yovanovitch on Twitter as she was testifying, prompting heavy criticism, including from Democrats who accused him of witness intimidation.
Mr. Sandy is the first budget official to speak with impeachment investigators, in defiance of a Trump administration directive not to cooperate.
At least three higher-profile Trump administration officials connected to the budget office have stiff-armed investigators: Russell T. Vought, the agency’s acting director; Michael Duffey, who helped carry out Mr. Trump’s directive to freeze the aid; and Mr. Mulvaney, who retains the title of budget director.
Mr. Sandy is the deputy associate director of the budget office’s national security division who once served as the agency’s acting director. Unlike others from his agency who have refused to show up, he is a career official, not a political appointee of Mr. Trump’s. Records in the possession of House investigators indicate that Mr. Sandy signed paperwork on July 25 enforcing the hold, but that Mr. Duffey, a political appointee, signed such paperwork going forward.
Ahead of Mr. Sandy’s testimony, a senior Trump administration official complained that Democrats were “threatening” career officials with subpoenas and depositions without granting the agencies they work for the right to send a lawyer to take part. The official added that Democrats would be “sorely disappointed” when they could not substantiate “their latest false narrative.”
Congress allocated the security assistance funds on a bipartisan basis this year to help Ukraine in its continuing military conflict with Russia. The decision by Mr. Trump to hold up the money indefinitely in July alarmed officials across the government, including at the Defense and State Departments, where the aid is viewed as vital not just to Ukraine’s national security but also to that of the United States.
A Defense Department official has already testified that she and others involved in Ukraine policy raised concerns about whether the hold would present legal problems if it was not reversed. Other witnesses have described how the president’s most senior advisers, including the secretaries of state and defense, pushed him to unfreeze the aid in August.
The White House has maintained that Mr. Trump held up the money out of generalized concerns about corruption in Ukraine and a frustration that European governments were not providing more money to the nation.
But in October, Mr. Mulvaney cited a third reason: The aid, he said at a news conference, was withheld in part until Ukraine investigated an unsubstantiated theory that Ukraine, not Russia, was responsible for hacking Democratic Party emails in 2016. He later tried to walk back that statement.
Mr. Trump eventually released the hold in September after intense bipartisan pressure from Congress that he do so and after an anonymous whistle-blower had filed a complaint based in part on the aid. Republicans now point to that fact to argue that his behavior was not impeachable.
Saturday’s interview was expected to cap what was a remarkable week on Capitol Hill, where the House Intelligence Committee convened the first full-blown public impeachment hearings in two decades. A top State Department official, the United States’ top envoy to Ukraine and his predecessor, who was abruptly removed from her post by Mr. Trump amid a smear campaign, helped put a public face on the inquiry in describing what they say amounted to a shadow foreign policy. The testimony, though, appeared to have done little to bridge the stark divide between Democrats and Republicans over the significance of the events in question.
The deposition of Mr. Sandy came after investigators worked late Friday night interviewing David Holmes, an official from the United States Embassy in Kiev, who described a call he overheard in July in which Mr. Trump asked his ambassador to the European Union whether the Ukrainian president had committed to an investigation into former Vice President Joseph R. Biden Jr. that Mr. Trump had personally pressed him to conduct a day earlier.
As of Saturday morning, House Democrats, who control the inquiry, had not scheduled any additional private witness interviews. But there will be three days of public hearings in the coming week, featuring some of the biggest names ensnared in the inquiry.