Trump’s defence suffers terrible blow
Yes, Mr Trump pressured Mr Zelensky to launch an investigation into Hunter Biden, the son of his chief political opponent. And yes, he pushed for an investigation into supposed Ukrainian interference in the 2016 election. Those facts have been conceded.
But, the argument goes, Mr Trump did not link those investigations to military aid Ukraine desperately wanted from the United States.
No quid pro quo; no impeachment.
The Democrats have never accepted that argument. They say Mr Trump's conduct was already potentially impeachable, even without evidence of a quid pro quo arrangement.
But put that debate to one side for now. Whatever its merits, the "no quid pro quo" line has become the core of Mr Trump's impeachment defence, and that is why this week's events were so damaging.
The most important development came on Tuesday, when America's Ambassador to Ukraine Bill Taylor testified before Congress.
He provided a detailed timeline of the Trump administration's dealings with Ukraine. If you have a spare 10 minutes, it's worth reading his entire opening statement, but I'll give you the short version here.
Mr Taylor said America's diplomacy with Ukraine had been split between the "regular" channel, led by himself, and an "irregular" channel run by Mr Trump's personal lawyer, Rudy Giuliani.
He said Ukraine wanted two things from the United States - a meeting with Mr Trump at the White House, and military aid that had already been approved by Congress, which it needed to defend itself against Russia.
Mr Trump had delayed the release of that aid, confusing his military and diplomatic advisers, who did not understand why.
Meanwhile, Mr Trump wanted Mr Zelensky to publicly announce investigations into both the 2016 election and Burisma, the company which paid Hunter Biden an exorbitant salary to sit on its board.
"By mid-July it was becoming clear to me that the meeting President Zelensky wanted was conditioned on the investigation of Burisma and alleged Ukrainian interference in the 2016 US elections. It was also clear that this condition was driven by the irregular policy channel I had come to understand was guided by Mr Giuliani," Mr Taylor said.
A key figure in his testimony was America's Ambassador to the European Union, Gordon Sondland.
According to Mr Taylor, in early September Mr Sondland told one of Mr Zelensky's senior aides, Andriy Yermak, that "the security assistance money would not come until President Zelensky committed to pursue the Burisma investigation".
A few days after that conversation, on September 8, Mr Taylor spoke to Mr Sondland on the phone.
"He said he had talked to President Trump as I had suggested a week earlier (to try to convince him to release the aid), but that President Trump was adamant that President Zelensky, himself, had to 'clear things up and do it in public'," he testified.
The phrase "clearing things up" there is a reference to Mr Zelensky publicly announcing the investigations Mr Trump wanted.
"President Trump said it was a not a 'quid pro quo'. Ambassador Sondland said that he had talked to President Zelensky and Mr Yermak and told them that, although this was not a quid pro quo, if President Zelensky did not 'clear things up' in public we would be at a 'stalemate'," said Mr Taylor.
"I understood a stalemate to mean that Ukraine would not receive the much-needed military assistance. Ambassador Sondland said that this conversation concluded with President Zelensky agreeing to make a public statement."
There was a heck of a lot more to Mr Taylor's testimony, but those are the most relevant bits and pieces. Again, I'd encourage you to read the whole statement.
This testimony is so important because it's the first clear evidence that there was an explicit quid pro quo - that Mr Trump did not want to release the military aid or grant Mr Zelensky a White House visit unless he agreed to announce those two investigations.
It doesn't matter that Mr Trump and Mr Sondland are quoted as saying it was not a quid pro quo.
If I walk out of a shop without paying for something, but tell the cashier I'm not stealing, it's still theft. And if you tell a foreign leader there is no quid pro quo, but say you will only give them what they want if they give you what you want - I'm sorry, that's still a quid pro quo.
Republicans tried out a fallback position of sorts this week, claiming Ukraine was unaware the military aid was being delayed, and therefore a quid pro quo arrangement was impossible.
Mr Taylor's testimony also shot down that argument, because it showed Ukrainian officials were told about the situation. They knew the aid was being held up, and they knew what Mr Trump wanted them to do before he would release it.
So we are left with two possibilities here. Either Mr Taylor is quite blatantly lying, and the "no quid pro quo" argument stands, or he is telling the truth, and it is no longer viable.
It is hard for Mr Trump to argue the man he himself appointed as Ukraine Ambassador, who actually still serves in that role, is so biased against him that he would simply make up all this damning evidence. No doubt he'll try anyway.
But in an ominous sign, the President's Republican colleagues appear to be falling back to a new line of defence.
Instead of continuing with a substantive defence of Mr Trump's conduct, they are now focusing their efforts on criticism of the impeachment process itself.
More than 30 Republican lawmakers stormed into a secure meeting room on Wednesday - the day after Mr Taylor's testimony - dramatically disrupting a closed hearing with one of the impeachment inquiry's witnesses.
They said it was a protest against the secretive nature of the hearings.
"One of the cornerstones of American jurisprudence is due process - the right to confront your accuser, call witnesses on your behalf and challenge the accusations against you. None of this is occurring," Senator Lindsey Graham said afterwards.
It's a strange argument.
We are currently in the investigation phase of the impeachment process. Congress is gathering information. It's the equivalent of prosecutors pulling together evidence to determine whether they should proceed with a case.
You've surely watched enough TV cop dramas to know that process doesn't tend to happen in public.
The hearings in Congress are closed to stop witnesses from co-ordinating their testimony. It's also worth noting that Republicans from the relevant congressional committees are in the room, where they have every right to grill those witnesses.
The due process Mr Graham was talking about will come if the House of Representatives actually decides to approve articles of impeachment, setting up a trial in the Senate.
That's the equivalent of a court trial, where Mr Trump will get to confront the accusations against him and his Republican colleagues will be part of the jury.
Until this week, the lack of a clear quid pro quo with Ukraine was their best excuse to vote in his favour. And that defence is now crumbling.