"Explosive" closed-door testimony on Tuesday from William Taylor, a career diplomat who questioned the Trump administration's attempts over the summer to pressure Ukraine to investigate former Vice President Joe Biden, could begin a "sea change" in House Democrats' impeachment inquiry against the president, according to lawmakers who were in the room.

Taylor gave a detailed account to the House Oversight, Intelligence, and Foreign Affairs committees of the administration's "pervasive" attempts to convince Ukrainian officials to investigate Biden, his son Hunter's position at a Ukrainian oil company, and a debunked conspiracy theory regarding Ukrainian interference in the 2016 election.

"The body language of the people hearing it was 'holy shit,' seriously."

—Rep. Harley Rouda (D-Calif.)

In a July phone call with Gordon Sondland, ambassador to the E.U., Taylor was told that a failure of Ukrainian President Volodymyr to publicly announce that he would launch the investigation would set off a deterioration of U.S.-Ukraine relations. Zelensky would not be invited to the White House and military aid would be withheld, Sondland said, unless he made the statement.

"Amb. Sondland said, 'everything' was dependent on such an announcement, including security assistance,'" Taylor testified to committee members on Tuesday. "He said that President Trump wanted President Zelensky 'in a public box' by making a public statement about ordering such investigations."

Read Taylor's full opening statement here.

Nearly $400 million in U.S. military aid for Ukraine was put on hold around the time of the July 25 phone call between Trump and Zelensky that's at the center of the impeachment inquiry.

Rep. Tom Malinowski (D-N.J.) told Politico that Taylor provided "the most thorough accounting we've had of the timeline" and suggested the envoy had provided the committees with corroborating evidence.

"This testimony is a sea change," Rep. Stephen Lynch (D-Mass.) told Politico. "I think it could accelerate matters. This will, I think, answer more questions than it raises."

Gasps were reportedly heard from lawmakers throughout Taylor's testimony, which began after House Democrats issued a subpoena following the State Department's attempt to block Taylor from speaking.

"The body language of the people hearing it was 'holy shit,' seriously," Rep. Harley Rouda (D-Calif.), a member of the Oversight Committee, told Politico.

"This testimony is a sea change. I think it could accelerate matters. This will, I think, answer more questions than it raises."

—Rep. Stephen Lynch (D-Mass.)

"All I have to say is that in my 10 short months in Congress—it's not even noon, right—and this is my most disturbing day in Congress so far," Rep. Andy Levin (D-Mich.) told reporters. "Very troubling."

The progressive advocacy group Stand Up America said that following the testimony, Republicans in Congress are "out of excuses" and must vote to impeach Trump.

"Today's bombshell testimony proves without a shadow of a doubt that Donald Trump used military aid to pressure Ukraine into investigating a political rival," said Sean Eldridge, president of the group. "Republicans in Congress must do their jobs and uphold their oath to defend our Constitution by voting to impeach and remove Donald Trump."

"No one—including the criminal in the White House—is above the law," he added.