The impeachment inquiry against Donald Trump has heard some extraordinary testimony over the last month. From the first mention of Trump’s desired “deliverable” from Ukraine, successive layers of witnesses and documents have added to an indictment of the president’s conduct that only gets heavier, as Trump howls his defenses to the wind.

On Tuesday, things got even worse for Trump – much worse, as many saw it.

For almost 10 hours, William Taylor, a former military officer and career diplomat with the rank of ambassador under the last four presidents, spoke with congressional investigators about how the Trump administration has been conducting a two-track foreign policy in Ukraine, where Taylor is in charge of the US embassy.

We don’t yet know most of what was said. The current public record of the closed-door testimony comprises only a copy of Taylor’s 15-page opening statement – and the spectacle of the ashen faces of members of Congress as they filed out from the hearing.

“This testimony is a sea change,” congressman Stephen Lynch told reporters.

In his testimony, Taylor explained his discovery of an “irregular, informal policy channel” by which the Trump administration was pursuing objectives in Ukraine “running contrary to the goals of longstanding US policy”. What the “informal channel” wanted – and briefly obtained, Taylor said – was for the Ukrainian president to agree to go on CNN to announce an investigation of Joe Biden, whom Trump sees, perhaps mistakenly, as a top 2020 threat.

Bill Taylor, acting ambassador to Ukraine, testified of ‘an informal policy channel’ by which the Trump administration was pursuing objectives in Ukraine. Photograph: Alex Wroblewski/Getty Images

The Trump administration held up “much-needed military assistance” to Ukraine in an effort to extract the Ukrainian statement, Taylor said. “More Ukrainians would undoubtedly die without the US assistance,” he noted.

In a process scrambled so far by misleading Trump tweets and relying in part on anonymous witnesses, the testimony of Taylor, a Vietnam veteran respected in both parties with 50 years of public service behind him, landed as a potential game-changer. It was just the kind of testimony that seemed to answer even the most stubborn demands of Trump loyalists such as Senator Lindsey Graham for additional, definitive proof that Trump was turning the broad power of his office to his own narrow devices.

“If you could show me that, you know, Trump actually was engaging in a quid pro quo, outside the phone call, that would be very disturbing,” Graham said at the weekend.

The senator denied in a Fox News appearance Tuesday that Taylor had delivered such evidence. But Taylor added significant ballast to the allegation that Trump was attempting to extort Ukraine into ginning up bad news about Biden.

What Taylor added was a careful stitchwork of detail, describing who was working to extort the Ukrainians, how they were going about it, how their aims clashed with stated US policy, how the Ukrainians responded, and what people said to him about it at the time.

Taylor made clear he has the memos and other records to back up his story. And he exposed the slapstick clumsiness of the Trump flunkies working the “informal channel” – notably Gordon Sondland, the hotelier and Trump mega-donor turned ambassador.

“Ambassador Sondland tried to explain to me that President Trump is a businessman,” Taylor testified. “When a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before signing the check.”

But “the explanation made no sense”, Taylor argued. “The Ukrainians did not ‘owe’ President Trump anything, and holding up security assistance for domestic political gain was ‘crazy’.”

Reaction to Taylor’s testimony generally fell between shock and dumbfoundedness. “I cannot overstate how damaging this Ambassador Taylor testimony is to Trump,” tweeted Neal Katyal, the former acting solicitor general.

“Taylor’s statement is a completely devastating document,” wrote Susan Hennessey, the executive director of the Lawfare site. “I know they will find a way but it’s just impossible to imagine how Republicans in Congress will be able to defend this. It is well beyond what most assumed was the worst-case scenario.”

The White House issued a statement Tuesday night impugning Taylor, a Trump appointee, as part of a cadre of “radical unelected bureaucrats waging war on the constitution”. But the taller the evidence against him, the smaller Trump’s protests seemed.

Democratic senator Amy Klobuchar, a presidential candidate, challenged Republicans to take a stand. “After Diplomat Taylor’s testimony you can no longer question whether this happened,” she tweeted. “The question is if you choose to follow the law or be part of the cover-up.”

Trump huddled Tuesday night with members of his legal team, the Wall Street Journal reported, and he urged congressional Republicans to do more to rebut the impeachment inquiry. But there were reportedly no talking points, and no one knew quite what they were supposed to say, or whom to take that direction from.

Notably absent from the meeting of Trump’s advisors was Rudy Giuliani, whom Taylor describes as running the shadow operation in Ukraine. “The official foreign policy of the United States was undercut by the irregular efforts led by Mr Giuliani,” Taylor said. He described a seemingly free hand for Giuliani, whose foreign clients include or have included Ukraine-based antagonists of current and former US officials, to open and close diplomatic channels and to direct US policy as he pleased.

One of the weightiest impacts of Taylor’s testimony might have to do with the senior US officials it names. Taylor took his concerns about Trump’s alleged attempt to extort Ukraine, he said, to both national security adviser John Bolton and to secretary of state Mike Pompeo. Bolton, who has since resigned, reacted with outrage and frustration.

Pompeo, who is eyeing a US Senate bid in his home state of Kansas, apparently greeted Taylor’s warning with silence.

“This is not the story of corruption in Ukraine,” tweeted the political strategist David Axelrod. “It’s the story of corruption at the highest levels of the US government. It’s the story of extortion, with US military aid to a besieged ally held hostage to the president’s personal political project.”

Trump’s critics say the story is plain: that the president twisted the immense powers of his office to personal ends, in betrayal of constitution and country. When it comes time to prove it, Taylor’s testimony is likely to be front and center.