WASHINGTON — One stunning moment during a top diplomat's testimony this week may prove pivotal to the congressional impeachment inquiry and even led to gasps in the room, according to one source who was present.

It occurred when William Taylor, the lead U.S. envoy to Ukraine, described a video conference call in July with officials from the White House Office of Management and Budget. Even Republicans who were present expressed concern, the source said, because the call made a direct link between President Donald Trump and the withholding of military aid to Ukraine for political purposes.

One OMB official, who was not named by Taylor during his appearance on Tuesday before the House Intelligence Committee, informed those on the call that there was a hold on U.S. military aid. While that OMB official didn't know why the funds were being frozen, a second OMB aide, who was not on camera, said "the directive had come from the president to the chief of staff to OMB," according to Taylor. The call occurred one week before Trump's discussion with Ukrainian President Volodymyr Zelenskiy.

"There were audible sighs and 'ughs' (during Taylor's deposition) when that process was described," according to the source.

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One member of Congress who was in the room, Rep. Debbie Wasserman Schultz, D-Fla., said the moment made the connection clear between the withholding of aid and Trump's demand that Ukraine conduct an investigation that could implicate his political opponent, former Vice President Joe Biden.

"He drew a very direct line in a series of events he described as being President Trump's decision to withhold funds and refuse a meeting with Zelenskiy," Wasserman Schultz said, "unless there was a public pronouncement by him of investigations of Burisma."

Burisma is the Ukrainian energy firm for which Biden's son Hunter served as a board member.

Last week, the acting chief of staff, Mick Mulvaney, acknowledged during a news conference that the aid was held up as part of a quid pro quo, although he later insisted his words had been misreported by the press.

In a July phone call with Zelenskiy, Trump asked him for a "favor" and then asked for help in investigating both the origins of the investigation into 2016 Russian election interference, as well as an energy company tied to Hunter Biden.

Taylor could be a linchpin of any impeachment case against Trump and that sentiment was realized when he delivered his opening statement to the committee on Tuesday. The gravity of the moment and the realization of what is at stake in the impeachment probe was palpable in the room, according to multiple sources who were in attendance.

At one point, "one prominent (Republican) member who will go nameless turned to an aide and said, 'This isn't good,'" a person in the room said.

Taylor's testimony was by far the most heavily attended of those who have appeared before the committee to date and the secure space in the basement of the U.S. Capitol was standing-room-only, filled with staff members and at least 20 lawmakers from the three committees tasked with running the inquiry.

The room went silent during Taylor’s 45-minute opening statement, which he delivered in a steady yet confident tone, stopping just a few times to take a sip of water, according to a source in the room.

"You could hear a pin drop," one source said.

Typed on 15 pages, Taylor's opening statement outlined what he thought was "crazy" about withholding security aid until the Ukrainians helped with a U.S. political campaign. He gave a detailed timeline explicitly laying out instances of the U.S. withholding aid and access until the Ukrainian government complied.

"The room got super silent and heavy as he was reading his opening statement," a third person in the room, said. "Democrats (were) looking at each other in shock. Republicans not looking up from the paper."

Another critical moment in Taylor's testimony that drew disbelief occurred when he told congressional investigators that he was told that Ambassador Gordon Sondland told him that "everything" — nearly $400 million of military and security aid and a meeting at the White House between Zelenskiy and Trump — was dependent upon a public declaration by the Ukrainian leader that he commit to investigating Burisma and Ukrainian involvement in the 2016 election.

People in the room described Taylor as an extremely careful and credible witness who took "meticulous" notes during his tenure. He referred to them often, people in the room said.

Taylor is expected to be called back to testify in public as part of a second phase of the Democratic-led inquiry that will be critical to building public support for impeachment, according to one Democratic aide involved in the process.

Republicans have struggled to figure out a message since Taylor's testimony since it potentially undercuts Trump's argument that there was "no quid pro quo." Republicans interrupted a planned deposition on Thursday by storming the secure room where the depositions are taking place, which is a violation of House rules that say only members of the appropriate committees are able to listen to the depositions.

While Republicans' arguments are based mostly on process and not substance, their tactics on Thursday did successfully change the narrative away from Taylor's testimony.

Taylor reluctantly came out of retirement to replace the former Ambassador to Ukraine Maria "Masha" Yovanovitch after she was ousted at the direction of the president after Rudy Giuliani ran a smear campaign against her.

He told congressional investigators that he agreed to serve because he received a commitment from Secretary of State Mike Pompeo that the United States' position on Ukraine, which includes a strong defense of the former Soviet-bloc country, remains the same.