On Wednesday morning, an obscure millionaire from Portland who was such an amateur at international diplomacy that one of his colleagues compared him to a car careening down a mountain road without a G.P.S. or guard rails, came as close as anyone has yet to blowing up the Presidency of Donald Trump.

Gordon Sondland, a genial hotelier whose million-dollar contribution to Trump’s Inauguration committee bought him the ambassadorship to the European Union, did not hold back when he appeared before the House Intelligence Committee’s impeachment inquiry. The Ukraine scandal was Donald Trump’s, from start to finish, Sondland told the panel, and his top advisers were all aware of it, enablers and facilitators of his scheme to pressure the new Ukrainian President, Volodymyr Zelensky, to launch investigations that would help Trump politically. Sondland said he worked with Trump’s personal lawyer, Rudy Giuliani, to try to force Zelensky into probing former Vice-President Joe Biden and the Ukrainian role in the 2016 U.S. election, because the President told him to. “Everybody knew what we were doing and why,” Sondland said. “Everyone was in the loop.” Vice-President Mike Pence knew. Secretary of State Mike Pompeo knew. The chief of staff, Mick Mulvaney, knew. The national-security adviser, John Bolton, knew. But, in the end, this was Trump’s show, and the price of their service in his Administration was complicity, willing or otherwise, in his scheme.

“We followed the President’s orders,” Sondland said.

“Mr. Giuliani was expressing the desires of the President of the United States, and we knew that these investigations were important to the President,” Sondland said.

“Was there a ‘quid pro quo’? . . . The answer is yes,” Sondland said.

“As a Presidential appointee, I followed the directions of the President,” Sondland said.

The Ambassador seemed to relish his unlikely role at the center of the impeachment affair. He had real stage presence as he slowly and expressively read his statement. He had timing. He seemed unburdened, unruffled, adamant. He made clear his distaste for Giuliani, and for the very concept of pressuring Ukraine by withholding nearly four hundred million dollars in military aid and a White House meeting. If he was an agent of Trump’s plot, he portrayed himself as a reluctant one. “We chose the latter course not because we liked it but because it was the only constructive path open to us,” Sondland said.

In testimony Tuesday, the former National Security Council official in charge of Russia and Ukraine, Timothy Morrison, said that he had been briefed by his predecessor, Fiona Hill, about “the Gordon problem.” The professionals at the N.S.C. feared that he was a troublesome freelancer, a counterintelligence nightmare who gabbed freely on his insecure cell phone and bragged about access to the President that he might or might not have had. Sondland’s testimony on Wednesday was an extended rebuttal to that. There was no Gordon Problem, he argued. There was a Trump Problem.

By 10:15 A.M., when Sondland finished reading out his twenty-three-page opening statement, it was all over but the shouting, of which there would be much over the next seven hours. But that was later. For nearly forty-five minutes, the impeachment hearings were compelling enough to bring the relentless Washington news cycle to a halt, at least temporarily. There was no hate-tweeting from the Oval Office, no spin in the corridors of Capitol Hill. Just people listening, watching, wondering what it would mean.

The reaction was swift and overwhelming. A bombshell. Bigger than Watergate. A John Dean moment. The most consequential day of the impeachment inquiry. The most consequential day of the Presidency. “Ambassador Drops Bombs,” the Drudge Report said. “Followed President’s Orders.” Over on Fox News, the former independent counsel Ken Starr, scourge of President Bill Clinton, said it looked so bad that Republican senators should consider going to the White House to ask for Trump’s resignation. John Roberts, the network’s chief White House correspondent, tweeted that it was “a devastating day for Donald Trump.” A Democratic member of Congress who was there to hear Sondland testify texted me, “He reinforces the key point. Everyone knew. No one approved. But they did not rein him in. The only possible reason is that they knew he was doing what Trump wanted.” White House sources admitted that Trump was watching, but otherwise there was, at first, nothing but silence from the President and his defenders.

Everyone understood a political earthquake had occurred, but this is a capital that has lost its belief in the power of political earthquakes to move the immovable American public. Would this time finally be different? Skepticism seemed in order in a country where everyone has made up his mind about Trump one way or another, but, still, who could be sure? “The veneer has been torn away,” Adam Schiff, the House Intelligence chairman, told reporters during a short break a couple of hours into the hearing. Republicans were still not commenting, except for a snarky tweet from the official White House account castigating “Shifty Schiff” for holding a press conference. When he finally did speak, it was vintage Trump, who already has a long record of distancing himself from those who turn on him. “I don’t know him very well. I have not spoken to him much,” the President told reporters. “He seems like a nice guy, though.”

For a couple of hours, the Republicans seemed stunned, disorganized. They clearly had not anticipated the extent and nature of Sondland’s willingness to flip on the President. In the morning, before Sondland had spoken, Devin Nunes, the panel’s top Republican, had opened the proceedings unaware of what he was about to hear. He had greeted Sondland as a friendly witness, warning him that he was about to be “smeared.” But clearly he had not yet read Sondland’s shocking opening statement. When the smearing came, it would have to be Nunes’s side that did it.

Sondland had left them plenty of room for attack. He was a problematic witness, with a hazy recollection of events, no notes to rely on, and an amateur’s lack of familiarity with the high-stakes international intrigue on which he had embarked. His account was undoubtedly self-serving and probably not entirely complete. After his initial deposition, a few weeks ago, he had no choice but to significantly revise it to reckon with inconvenient facts that others had revealed, such as an overheard phone call in a Kyiv restaurant with Trump that he never mentioned at all and which will be the subject of Thursday’s impeachment hearing. (“My memory, admittedly, has not been perfect,” he said.) What’s more, Sondland’s most dramatic claim, that Trump had conditioned U.S. military assistance on the investigations he sought, was his own “presumption,” Sondland testified, not something he had been explicitly told by Trump. In one sharp exchange in the afternoon, Sondland said he resented the implication made by a Democratic congressman, Sean Patrick Maloney, that he had been less than forthcoming. Maloney was having none of it. “With all due respect, sir, we appreciate your candor,” Maloney said, “but let’s be clear what it took to get it,” noting acerbically that this was Sondland’s “third try” at testifying, after his initial closed-door deposition and revised written statement.