WASHINGTON >> William Taylor, the United States’ top diplomat in Ukraine, told impeachment investigators privately today that President Donald Trump held up vital security aid for the country and refused a White House meeting with Ukraine’s leader until he agreed to make a public pronouncement pledging to investigate Trump’s political rivals.

In testimony that Democrats in attendance called the most damaging account yet for the president, Taylor provided an “excruciatingly detailed” opening statement that contradicted in blunt and unsparing terms the quid-pro-quo pressure campaign that Trump and his allies have long denied.

When he objected to that effort, Taylor said in his opening statement obtained by The New York Times, one of the president’s allies sought to explain Trump’s actions by noting that he was a businessman, and saying that, “when a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before signing the check.”

Taylor’s testimony directly contradicted repeated assertions by Trump and his Republican allies that there was never a quid pro quo involving investigations into Burisma, a Ukrainian gas company that employed Hunter Biden, the son of former Vice President Joe Biden, and other Democrats.

That is not true, Taylor told the committee. He said the president had explicitly made it clear that Ukraine’s president, Volodymyr Zelenskiy, would not be invited to the White House or secure much-needed security aid unless the Ukrainian leader made a public announcement that his country would start the investigations that Trump so badly wanted.

Taylor testified that he was told of Trump’s demands for investigations during a telephone call with Gordon Sondland, the U.S. ambassador to the European Union and a Trump campaign donor, who Taylor described as part of a “highly irregular” diplomatic effort aimed at pressuring Ukraine.

“Ambassador Sondland said that ‘everything’ was dependent on such an announcement, including security assistance,” Taylor told lawmakers. “He said that President Trump wanted President Zelenskiy ‘in a public box’ by making a public statement about ordering such investigations.”

Taylor added that: “During that phone call, Ambassador Sondland told me that President Trump had told him that he wants President Zelenskiy to state publicly that Ukraine will investigate Burisma and alleged Ukrainian interference in the 2016 U.S. election,” Taylor said.

One lawmaker described the testimony as drawing a “direct line” between U.S. foreign policy and Trump’s own political goals.

Rep. Debbie Wasserman Schultz, D-Fla., who sat in on the deposition as a member of the House Oversight and Reform Committee, said that Taylor relied in part on detailed “notes to the file” that he had made as he watched the pressure campaign unfold. His testimony shed new light on the circumstances around a previously revealed text message in which Taylor wrote to colleagues that he thought it was “crazy to withhold security assistance for help with a political campaign.”

He “drew a very direct line in the series of events he described between Trump’s decision to withhold funds and refuse a meeting with Zelenskiy unless there was a public pronouncement by him of investigations of Burisma and the so-called 2016 election conspiracy theories,” Wasserman Schultz said.

In his statement, Taylor placed the reason for the hold on Ukrainian aid directly on Trump, relating a July 18 call with the Office of Management and Budget, where a staff member said the directive had come from the president to the acting White House chief of staff, Mick Mulvaney.

“In an instant I realized one of the key pillars of our strong support for Ukraine was threatened,” Taylor said in his testimony.

The intelligence whistleblower’s complaint that prompted the impeachment inquiry said that Trump’s effort to pressure Zelenskiy to open an investigation of Burisma was part of a concerted effort to use the power of his office to enlist foreign help in the 2020 election. Taylor was the latest in a string of career diplomats and current and former administration officials who have defied a White House blockade of the impeachment inquiry and submitted to closed-door depositions with investigators digging into whether Trump abused his power to pressure Ukraine to investigate his political adversaries.

The president sought to discredit the inquiry with attention-grabbing rhetoric early today as Taylor made his way to Capitol Hill to testify, comparing the impeachment investigation against him to a “lynching.” His comment on Twitter drew bipartisan outrage in public as the ambassador made his case behind closed doors.

Wasserman Schultz said that in addition to referencing his notes, Taylor “had very specific recall of things,” including what she said were “meetings, phone calls, what was said.”

Several Democrats who participated in Taylor’s questioning described his testimony. Rep. Ted Lieu, D-Calif., shook his head after exiting the deposition, saying “what he said was incredibly damning to the president of the United States.”

Wasserman Schultz called it “one of the most disturbing days” she has had in Congress, and added: “I have not seen a more credible witness than this.”

Republicans accused Democrats of exaggerating, but they declined to share details of the testimony.

“I don’t know that any of us, if we are being intellectually honest, are hearing revelations that we were not aware of,” said Rep. Mark Meadows, R-N.C. “The bottom line is no one has yet to make the case for why the aid was withheld or even if the Ukrainians knew about it.”

Still, by Democrats’ account, Taylor’s testimony provided the most extensive picture yet of the scope of the president’s effort to pressure Ukraine and the players who were involved in the effort on Trump’s behalf.

“It’s like if you had a big, 1,000-piece puzzle on a table,” Wasserman Schultz said. “This fills in a lot of pieces of the puzzle.”

Taylor became a star witness in the Democratic impeachment probe after a colleague, Kurt Volker, the special envoy to Ukraine, revealed texts they exchanged. In some of the text chains, as Taylor expressed his concerns about an apparent quid pro quo, Sondland sought to take the conversation offline, telling Taylor to “call me.”

Wasserman Schultz said Taylor provided new context for and details about the exchanges with Sondland, including a phone call that occurred after the texts.

In his lengthy opening statement and in questioning afterward, Taylor laid out a meticulous timeline of events during his time in the administration.

Taylor’s habit of keeping notes throughout his tenure has given the inquiry a boost, allowing him to recreate crucial conversations and moments even as the administration seeks to block Congress from reviewing documents related to its dealings with Ukraine.

Taylor has shared his notes with the State Department but has not produced copies of them for lawmakers conducting the impeachment inquiry, a person familiar with his testimony said.

The State Department objected to Taylor’s appearance before the committee, according to an official working on the impeachment inquiry. In response, the House Intelligence Committee issued a subpoena this morning to compel his testimony, and Taylor complied, according to the official.