Acting U.S. Ambassador to Ukraine William B. Taylor Jr. tied U.S. military aid to Kyiv to an investigation of Vice President Joe Biden in closed door testimony Tuesday before House impeachment investigators.

Taylor, a seasoned diplomat, contradicted President Trump’s denial of allegations he tried to use release of nearly $400 million in military aid as leverage in efforts to get Ukraine to investigate Biden and his son, Hunter, as well as Ukraine’s role in the 2016 presidential election.

“It was just the most damning testimony I’ve heard,” Rep. Debbie Wasserman Schultz, D-Fla., told the Washington Post.

“He drew a very specific direct line from President Trump to the withholding of foreign aid and the refusal of a meeting” with Zelensky.

Rep. Stephen Lynch, D-Mass., said Taylor’s testimony represented a “sea change” in the impeachment inquiry and could accelerate it.

In a 15-page opening statement, Taylor said he told other officials it was “crazy” to make the assistance contingent on the investigations. He also said Ukrainian officials were livid they had been blindsided by Trump’s release of a rough transcript of a July 25 call Trump made to Ukraine President Volodymyr Zelensky in which Trump asked for a “favor.”

Taylor testified he was alarmed by “weird” secondary diplomatic channels involving U.S. officials at the behest of Trump personal attorney Rudy Giuliani.

“In August and September of this year, I became increasingly concerned that our relationship with Ukraine was being fundamentally undermined by an irregular informal channel of U.S. policymaking and by the withholding of vital security assistance for domestic political reasons,” Taylor said.

Taylor said Trump told U.S. Ambassador to the EU Gordon Sondland to tell Zelensky he must “go to a microphone and say he is opening investigations of Biden and 2016 election interference, and that President Zelensky should want to do this himself.” He said he learned of this from Tim Morrison, the White House official in charge of Europe, who notified then-national security adviser John Bolton and National Security Council lawyers.

Taylor testified under subpoena after the State Department attempted to block his appearance.

He told lawmakers Bolton opposed Trump’s call to Zelensky and urged Taylor to make his concerns known to Secretary of State Mike Pompeo.

"I wrote and transmitted such a cable on Aug. 29, describing the ‘folly’ I saw in withholding military aid to Ukraine at a time when hostilities were still active in the east and when Russia was watching closely to gauge the level of American support for the Ukrainian government," Taylor said. "I told the secretary that I could not and would not defend such a policy.”