Donald Trump's lawyers have built his impeachment defence on a house of cards, and it is starting to look worryingly unstable.
Legal experts in the United States have criticised Trump's team this week for basing their case on a denial that there was any "quid pro quo" in the President's dealings with Ukraine.
Here's the quick version of that argument.
At the same time that Trump was pressuring Ukrainian President Volodymyr Zelensky to open an investigation into his political opponent, Joe Biden, he was withholding $391 million of military aid for Ukraine.
According to Trump – and his legal team – those two things were not connected. There was no suggestion from the President that the aid would stay frozen unless Ukraine met his demand for the investigation.
In other words, there was no quid pro quo.
"Not a single witness testified that the President himself said that there was any connection between any investigation and security assistance, a presidential meeting, or anything else," Trump's personal lawyer Jay Sekulow told the impeachment trial in the US Senate this week, presenting the case for the defence.
The problem is, there is a witness who is willing to say exactly that.
John Bolton, who served as Trump's national security adviser until the President fired him last September, has indicated he will testify at the trial if the Senate decides to subpoena him.
Bolton also happens to be writing a book about his time working in the White House. And earlier this week, details from the manuscript were reported in the media.
Among those details was a claim from Bolton that Trump personally told him he wanted to continue the aid freeze until Ukraine co-operated with his push for an investigation of Biden.
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That claim directly contradicts the President's insistence there was no quid pro quo – and completely undermines the key plank of his defence team's impeachment argument.
Now some Republican senators, who have previously resisted the Democrats' call for witnesses to appear during the trial, seem to be wavering.
"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Senator Mitt Romney said after the details from Bolton's manuscript leaked.
"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," said fellow Republican Susan Collins.
Only four Republican senators need to vote with the Democrats to create a majority in favour of allowing witnesses.
The bulk of Republicans remain opposed to the idea, but observers are now predicting as many as "five to 10" of them could support it, and Senate Majority Leader Mitch McConnell has told colleagues he does not yet have enough votes to block it.
Today, Fox News legal analyst and former judge Andrew Napolitano said he had warned Trump's team against pursuing their current impeachment strategy, and now Bolton's claim would go "to the core of the case".
"Many of us said to the President's people, after the impeachment, don't argue the facts. Concede the facts. Just argue that it does not constitute impeachable behaviour," he said.
"That is at least a plausible argument.
"But now they've chosen to argue the facts, and without any witnesses supporting their side of the facts. And now that this witness at the core of everything, who had personal conversations with the President, has come forward with a version 180 degrees from theirs, I think they're stuck with this.
"I think many Republicans are now going to say, 'We have to hear from Bolton.'"
Napolitano said if Democrats succeed in calling Bolton as a witness, it could open a "Pandora's box" and extend the trial – which Trump's team had hoped to finish quickly – by weeks.
"First, it's going to delay things. It's going to extend things. I believe it's going to open Pandora's box. It's going to open calls to lots of other witnesses to either rebut John Bolton or to reinforce John Bolton," he said.
"Suppose, for example, John Bolton relates a conversation that only he and the President had. How are the President's people going to rebut that without the unthinkable – putting the President on the stand?"
Trump has already strenuously denied Bolton's account.
"I never told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump tweeted this week.
"If John Bolton said this, it was only to sell a book."
But saying something on Twitter is not the same as saying it under oath, and under penalty of perjury.
Former federal prosecutor Andrew McCarthy, who is usually inclined to defend Trump, also issued a scathing assessment of the President's legal strategy this week.
His argument was similar to Napolitano's – that Trump's team should have conceded the facts of the case and focused on arguing that his conduct was simply not bad enough to warrant impeachment.
"You always want the foundation of your defence to be something that is true, that you are sure you can prove, and that will not change. Instead, the President and his team decided to make a stand on ground that could not be defended, on facts that were unfolding and bound to change," McCarthy wrote for National Review.
"For months, I've been arguing that the President's team should stop claiming there was no quid pro quo conditioning the defence aid Congress had authorised for Ukraine on Kyiv's conducting of investigations the President wanted.
"Trials and impeachment itself are unpredictable. You don't know what previously undisclosed facts might emerge during the trial that could turn the momentum against you. So you want to mount your best defence, the one that can withstand any damaging new revelations."
Trump's team did finally pivot as it finished its presentation today, arguing the allegations against the President would not be impeachable even if they were true.
"Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offence," said another of his lawyers, Alan Dershowitz.
"That is clear from history, that is clear from the language of the constitution. You cannot turn conduct that is not impeachable into impeachable conduct, simply by using words like 'quid pro quo' and 'personal benefit'.
"Quid pro quo, alone, is not a basis for abuse of power. It's part of the way foreign policy has been operated be presidents since the beginning of time."
The chances of Trump actually being convicted and removed from office at the end of this process remain extremely low.
Removal requires a two-thirds majority of the Senate, which means 20 Republican senators would need to vote against their own President. That is, quite simply, not going to happen.
But with the presidential election looming in November, the political optics of the trial do matter. The difference between a party-line vote, with a small majority of senators voting to acquit Trump, and a bipartisan majority voting to convict him is massive.
Many senators have their own re-election battles to fight, and enough of them have repeated the "no quid pro quo" line to justify their defence of Trump that public testimony from Bolton could be deeply awkward for them. Here are a few samples.
Lindsey Graham: "If you could show me that Trump actually was engaging in a quid pro quo, outside the phone call, that would be very disturbing."
Tom Cotton: "What's apparent is there is no quid pro quo that the President asked for anything in return for US aid to Ukraine."
Kevin Cramer: "It would be troubling if any president did a quid pro quo with tax dollars, but so far we don't have evidence that's happened."
Chuck Grassley: "There was no quid pro quo. You'd have to have that if there was going to be anything wrong."
Ted Cruz: "The Democrats were breathlessly on TV saying, 'You're going to see an illegal quid pro quo. It's going to prove that. Well, you look at the transcript, there's no illegal quid pro quo in that transcript."
Shelley Moore Capito: "I don't see a quid pro quo in here. I see a conversation between two leaders that is pretty broad-ranging. I just don't think this rises to impeachable."
Rob Portman: "I don't see the quid pro quo that the Democrats are claiming. In fact, I actually believe that if Speaker Pelosi and the Democrats had taken another 24 hours to make their decision and actually looked at the facts, which is the transcript itself, they might not have moved forward, because there is no quid pro quo."
Bill Cassidy: "Nothing in the transcript supports Democrats' accusation that there was a quid pro quo."
Mike Braun: "If you read the transcript closely there is no quid pro quo."
You can see how enthusiastically Trump's defenders have adopted the "no quid pro quo" argument. In hindsight, that may prove to have been a grave error.