WASHINGTON — The top U.S. diplomat in Ukraine, Bill Taylor, told members of Congress Tuesday that President Donald Trump directed officials to tie foreign aid to Ukraine to demands that the country open an investigation into the Biden family and the 2016 election, a potentially serious blow to Trump's repeated denials of a quid pro quo.

According to a copy of his opening statement provided to NBC News, Taylor said that E.U. Ambassador Gordon Sondland told him that while Trump was not requesting a "quid pro quo," he insisted Ukrainian President Volodymyr Zelenskiy publicly announce investigations into the Bidens and matters relating to the 2016 presidential election.

Members of the House Intelligence, Oversight and Foreign Affairs committees questioned Taylor about conversations he had with other American diplomats about the Trump administration's policy toward Ukraine.

Taylor told the committees that "it was becoming clear" to Taylor as early as July that nearly $400 million of military aid was being withheld on the condition that Zelenskiy commit to investigating the Burisma energy company as well as a conspiracy theory about alleged Ukrainian interference in the 2016 election.

Taylor said that Sondland told him in September that "everything," meaning military aid and a meeting with Trump in Washington, was dependent on Zelenskiy making a public statement committing to order the investigations.

Sondland "said that President Trump wanted President Zelenskiy in 'a public box' by making a public statement about ordering such investigations," Taylor testified.

Taylor said his concerns grew throughout the summer. He said he raised concerns to Secretary of State Mike Pompeo and considered resigning. He also raised objections to officials at the National Security Counsel at the White House, Taylor testified.

In his 15-page opening statement, Taylor also charged that much of the Trump administration's dealings with Ukraine circumvented normal diplomatic channels. Taylor said that when he took over the position, "I found a confusing and unusual arrangement for making U.S. policy towards Ukraine. There appeared to be two channels of U.S. policy-making and implementation, one regular and one highly irregular."

"The push to make President Zelenskiy publicly commit to investigations of Burisma and alleged interference in the 2016 election showed how the official foreign policy of the United States was undercut by the irregular efforts led by [Trump's personal lawyer Rudy] Giuliani," Taylor added later.

Democrats emerging from the day-long deposition Tuesday, which began at 9:30 a.m. and lasted more than nine hours, said that Taylor’s testimony offered a "disturbing" portrayal of Trump's Ukraine dealings. Members described Taylor's testimony as crucial, saying that he not only filled in many of the holes created by previous testimony and depositions but also drew a "direct line" between the president's demand for an investigation by the Ukrainians into his political rivals and U.S. military aid.

In his testimony Taylor said he was told that the president insisted there was no quid pro quo, but Taylor painted a picture of a series of events that would be defined as such.

"I do not know how you would listen to today's testimony by the ambassador, Ambassador Taylor, and draw any other conclusion, except that the president abused his power and withheld foreign aid," said Rep. Debbie Wasserman Schultz, D-Fla. "It's a direct line."

Speaking to reporters after hearing Taylor’s testimony, Rep. Karen Bass, D-Calif., chairwoman of the Congressional Black Caucus, called the diplomat’s remarks "shocking" and said "it was so clear that this was what was going to be required for foreign assistance which is just so egregious."

Two Democrats also said that Taylor took "meticulous" personal notes but those have not yet been handed over to the committee.

After departing the closed-door deposition a few hours in, freshman Rep. Andy Levin, D-Mich., told reporters that it was his "most disturbing day in the Congress so far ... very troubling."

Republicans said little, or played down Taylor’s testimony. "Nothing new here," Rep. Mark Meadows of North Carolina said.

"President Trump has done nothing wrong — this is a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution," said White House Press Secretary Stephanie Grisham. "There was no quid pro quo. Today was just more triple hearsay and selective leaks from the Democrats’ politically motivated, closed-door, secretive hearings."

As part of text messages between Taylor, Sondland, and then-special envoy to Ukraine Kurt Volker, Taylor expressed concern about why U.S. military assistance for Ukraine was held up by the White House.

"As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland on Sept. 9, according to text messages provided to Congress by Volker and released by the committees involved in the inquiry.

Democrats have pointed to the text exchange, a critical piece of the impeachment investigation, as part of a burgeoning body of proof that there was a quid pro quo involved between Trump and Zelenskiy in which Trump held up the military aid in exchange for Ukraine agreeing to conduct probes that would be politically advantageous to Trump.

"He was very distressed," according to a source who witnessed firsthand Taylor’s experiences dealing with the Zelenskiy and Trump administrations between May and September of this year.

Sondland testified to lawmakers last week that he wasn’t aware of the link between the request to investigate energy company Burisma and former Vice President Joe Biden’s son.

Sondland refused to answer many questions in his testimony last week, according to multiple lawmakers who listened to his testimony. Sondland told Taylor, who had raised concerns about withholding military aid in exchange for a political campaign, in a text message on Sept. 9 after speaking to the president that the president was clear that there is "no quid pro quo."

As of Tuesday, Sondland has not been asked to come and testify again before the House, according to a person with knowledge of Sondland’s plans. But if they do ask, he is willing to do so, that person said.

Taylor came out of retirement in June to serve as chargé d'affaires in Kyiv after Marie Yovanovitch was abruptly recalled as ambassador to Ukraine in the spring.

Yovanovitch, Sondland and Volker are among the key witnesses who have already testified in the impeachment inquiry. The White House has vowed not to cooperate with what it called an invalid investigation, with the House exercising its subpoena powers in response.

Yovanovitch, a career diplomat, told impeachment investigators in a deposition that lasted more than nine hours that Trump had personally pressured the State Department to remove her, even though a top department official assured her that she had "done nothing wrong."

According to an official working on the investigation, Taylor was issued a subpoena Tuesday morning by the House Intelligence Committee to compel his deposition in light of Trump administration efforts to block or limit his appearance.

A person who was in the room for Taylor's testimony said that Taylor testified that no one in the administration reviewed his opening statement before he delivered it.