In the four weeks since Democrats opened an impeachment inquiry into Donald Trump, there has been no testimony as damning to the president as that of Bill Taylor, a career diplomat now serving as acting ambassador to Ukraine, who described a sweeping shadow campaign by Trump and his allies to suborn foreign interference in the 2020 election. “It is a rancorous story about whistle-blowers, [Rudy] Giuliani, side channels, quid pro quos, corruption, and interference in elections. In this story, Ukraine is an object,” Taylor told lawmakers in his opening statement Tuesday.

Over the course of more than nine hours, he meticulously outlined Trump’s efforts to pressure Ukrainian President Volodymyr Zelensky into publicly announcing investigations into Hunter Biden, the former vice president’s son, and an unfounded conspiracy theory that Ukraine framed Russia for election meddling in 2016. And Taylor confirmed that the White House made these investigations a precondition for the release of nearly $400 million in military aid to help Ukraine counter an ongoing Russian invasion—the quid pro quo Trump and his allies have been denying since the story broke.

“This was a game changer,” a senior Democratic congressional aide told me. “Ambassador Taylor looks like he’ll end up being the star witness for the prosecution in this impeachment trial of a mob boss president. He not only exposed the full scope of corruption in the White House, but he left no doubt that President Trump was deliberately extorting a foreign country to try and get them to interfere on his behalf in the 2020 election. The president has to know the walls are closing in.”

Even before Taylor appeared before Congress, Democrats believed they already had enough evidence to impeach. “The strongest evidence of wrongdoing is the call memorandum that captures the president’s own words,” Congressman Sean Patrick Maloney, who sits on the House Intelligence Committee, told me last week, referring to the rough readout of Trump’s July 25 phone call with President Zelensky. “The other witnesses and documents have been reinforcing and corroborating the central set of facts. But we’ve known them from the beginning: that the president cooked this up, that he engaged in it personally, that he did it willfully, knowingly. It’s a sweeping abuse of power.”

Even so, Taylor—who testified that the directive to block the military aid came directly from Donald Trump and that E.U. Ambassador Gordon Sondland had conveyed the president’s order that “everything” was contingent on Ukraine launching investigations—was critical to corroborating a fact pattern that will provide the basis for impeachment. “There’s a lot of information. It fills in a lot of blanks. It helps you understand the broader scope of this scheme,” said Congressman Mike Quigley, another member of the intelligence panel. Perhaps most important, he explained, each piece of evidence has reinforced the others. “Nothing I have heard publicly, privately, or whatever, has done anything but corroborate and strengthen that which is in the public record—the whistle-blower’s complaint, the White House version of a transcript [of Trump’s July 25 call with Zelensky].”

The question is: Will Taylor’s testimony move Republicans out of Trump’s corner? A new Quinnipiac poll, published on Wednesday, found that support for impeachment has risen to 93% among Democrats and 58% among independents. But few Republican lawmakers, save for iconoclasts like Rep. Justin Amash, who left the GOP earlier this year, and Rep. Francis Rooney, who is retiring, have suggested they would be open to impeachment. Speaking to Fox News on Tuesday night, House Minority Leader Kevin McCarthy said that a fellow lawmaker, Rep. John Ratcliffe, had destroyed Taylor’s testimony “in 90 seconds”—though he declined to say how. “We can’t really talk about it,” he offered, repeating that there was no quid pro quo. “Everything is second-, third-, and fourth-hand information.” A White House spokeswoman, meanwhile, described Taylor as a “radical unelected bureaucrat.”