WASHINGTON — A critical witness in the impeachment inquiry offered Congress substantial new testimony this week, revealing that he told a top Ukrainian official that the country likely would not receive American military aid unless it publicly committed to investigations President Trump wanted.

The disclosure from Gordon D. Sondland, the United States ambassador to the European Union, in four new pages of sworn testimony released on Tuesday, confirmed his involvement in laying out a quid pro quo to Ukraine that he had previously not acknowledged. The issue is at the heart of the impeachment investigation into Mr. Trump, which turns on the allegation the president abused his power to extract political favors from a foreign power.

Mr. Trump has consistently maintained that he did nothing wrong and that there was no quid pro quo with Ukraine.

Sign Up For the Morning Briefing Newsletter

Mr. Sondland’s testimony offered several major new details beyond the account he gave the inquiry in a 10-hour interview last month. He provided a more robust description of his own role in alerting the Ukrainians that they needed to go along with investigative requests being demanded by the president’s personal lawyer Rudolph W. Giuliani. By early September, Mr. Sondland said, he had become convinced that military aid and a White House meeting were conditioned on Ukraine committing to those investigations.

The additions Mr. Sondland made to his testimony were significant because they were the first admission by a senior figure who had direct contact with Mr. Trump that the military aid for Ukraine was being held hostage to the president’s demands for investigations into his political rivals. A wealthy Oregon hotelier who donated to the president’s campaign and was rewarded with the plum diplomatic post, Mr. Sondland can hardly be dismissed as a “Never Trumper,” a charge that Mr. Trump has leveled against many other officials who have offered damaging testimony about his conduct with regard to Ukraine.

As such, Mr. Sondland’s new, fuller account is likely to complicate Republicans’ task in defending the president against the impeachment push, effectively leaving them with no argument other than that demanding a political quid pro quo from a foreign leader may be concerning, but — in the words of Mr. Trump himself — is not “an impeachable event.”

Mr. Sondland had said in a text message exchange in early September with William B. Taylor Jr., the top American diplomat in Ukraine, that the president had been clear there was no quid pro quo between the aid and investigations of former Vice President Joseph R. Biden Jr., his son and other Democrats. But Mr. Sondland testified last month that he was only repeating what Mr. Trump had told him, leaving open the question of whether he believed the president. His addendum suggested that Mr. Sondland was not completely forthcoming with Mr. Taylor, and that he was, in fact, aware that the aid was contingent upon the investigations.

In his updated testimony, Mr. Sondland recounted how he had discussed the link with Andriy Yermak, a top adviser to President Volodymyr Zelensky of Ukraine, on the sidelines of a Sept. 1 meeting between Vice President Mike Pence and Mr. Zelensky in Warsaw. Mr. Zelensky had discussed the suspension of aid with Mr. Pence, Mr. Sondland said.

“I said that resumption of the U.S. aid would likely not occur until Ukraine provided the public anticorruption statement that we had been discussing for many weeks,” Mr. Sondland said in the document, which was released by the House committees leading the inquiry, along with the transcript of his original testimony from last month.

The new information surfaced as the House committees also released a transcript of their interview last month with Kurt D. Volker, the former special envoy to Ukraine. Rushing to complete their final round of requests for key witnesses before they commence public impeachment hearings, the panels also scheduled testimony on Friday by Mick Mulvaney, the acting White House chief of staff. And two more administration witnesses who had been scheduled to testify on Tuesday — Michael Duffey, a top official at the White House budget office, and Wells Griffith, a senior aide to the energy secretary, Rick Perry — failed to appear.

In his new testimony, Mr. Sondland said he believed that withholding the aid — a package of $391 million in security assistance that had been approved by Congress — was “ill-advised,” although he did not know “when, why or by whom the aid was suspended.” But he said he came to believe that the aid was tied to the investigations.

“I presumed that the aid suspension had become linked to the proposed anticorruption statement,” Mr. Sondland said.

In his closed-door interview last month, Mr. Sondland portrayed himself as a well-meaning and at times unwitting player who was trying to conduct American foreign policy with Ukraine with the full backing of the State Department while Mr. Giuliani, Mr. Trump’s lawyer, repeatedly inserted himself at the behest of the president. He also said repeatedly that he could not recall the events under scrutiny, including details about the Sept. 1 meeting, according to the 375-page transcript of his testimony.

But some Democrats painted him as a lackey of Mr. Trump’s who had been an agent of the shadow foreign policy on Ukraine, eager to go along with what the president wanted. They contended that Mr. Sondland had deliberately evaded crucial questions during his testimony.

And other witnesses have pointed to him as a central player in the irregular channel of Ukraine policymaking being run by Mr. Trump and Mr. Giuliani, and the instigator of the quid pro quo strategy. Read more at MSN