The former national security adviser John Bolton proved over the weekend why he's President Donald Trump's worst nightmare as Trump battles a snowballing Senate impeachment trial.

Citing an unpublished manuscript of Bolton's upcoming book, The New York Times reported on Sunday that Bolton claimed Trump personally told him last year that he would withhold military aid from Ukraine until it launched a politically motivated investigation into former Vice President Joe Biden.

The revelation shatters Trump's biggest defense in the impeachment inquiry: that there are no firsthand witnesses who can directly confirm that he engaged in a quid pro quo regarding Ukraine.

It also boxes in Republican senators who have been against calling new witnesses in Trump's trial.

Trump disputed Bolton's reported claim, tweeting that he had no such conversation with the former national security adviser. But Bolton is said to be a meticulous note-taker, significantly bolstering his credibility as a firsthand witness.

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In the months since Congress launched its impeachment inquiry into President Donald Trump, there has been widespread speculation that the former national security adviser John Bolton could be Trump's worst nightmare if he decided to testify in the House's impeachment hearings or the Senate's trial.

This weekend, Bolton proved why.

The New York Times on Sunday published a bombshell report that in an unpublished manuscript for his upcoming book, Bolton claimed that Trump personally told him last year that he would withhold military aid to Ukraine until Ukrainian President Volodymyr Zelensky acceded to his demands for politically motivated investigations targeting former Vice President Joe Biden and the Democratic Party.

Specifically, Trump demanded that Zelensky investigate unfounded allegations of corruption against Biden, a frontrunner for the 2020 Democratic presidential nomination, and his son Hunter, related to the younger Biden's employment on the board of the Ukrainian natural-gas company Burisma Holdings. The president also wanted Zelensky to investigate a discredited conspiracy theory suggesting Ukraine interfered in the 2016 US election.

The president and his allies have repeatedly claimed that his decision to freeze Ukraine's aid had nothing to do with his demands for investigations. Instead, they said, Trump was concerned about European burden-sharing and corruption in Ukraine.

"This is a firsthand account of a conversation with Trump himself, from a highly respected public servant in the conservative foreign policy community," Jens David Ohlin, a vice dean at Cornell Law School and an expert in criminal law, told Insider. "It is devastating to Trump's legal defense."

Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department, echoed that view.

Bolton's revelation "directly contradicts Trump's absurd defense on the facts," he said. "The Senate can hear the story as part of their responsibilities, or they can hear it on Bolton's book tour."

So far, the Republican-controlled Senate has refused to vote on whether to call new witnesses, like Bolton, in the president's impeachment trial. But his revelation that the president, in a face-to-face conversation, confirmed a quid pro quo — security assistance in exchange for a personal, political favor — makes it nearly impossible for the GOP to ignore.

"There is no excuse for not seeking Bolton's testimony," Cramer said. He added that Trump could invoke executive privilege to bar his former national security adviser from appearing before the Senate, in which case it would go to the courts.

Any president can reasonably expect that they can have candid conversations with advisers without the substance of those conversations being publicly revealed. However, Cramer said, executive privilege doesn't extend to potential abuse of power — one of the articles of impeachment against Trump — and when the evidence is stacked against the president.

Indeed, more than a dozen current and former US officials have testified to Congress, under oath, about Trump's months-long effort to pressure Ukraine to cave to his demands while withholding military aid and dangling a White House meeting Zelensky desperately sought.

In the White House's summary of the July 25 phone call at the center of the impeachment inquiry, Trump repeatedly asked his Ukrainian counterpart to deliver the investigations he wanted. He made the demands immediately after Zelensky indicated Ukraine was ready for more US military aid.

"It would be easier if Trump simply asked for an envelope full of cash," Cramer said. "He didn't need the cash. He needed the announcement of an investigation."

Bolton's reported claim about his conversation with Trump, which he said took place last August, also shatters the key defense the president's lawyers have put forward in his impeachment trial: There are no firsthand witnesses who can testify that Trump himself confirmed a quid pro quo with Ukraine.

"Not a single witness testified that the president himself said that there was any connection between any investigations and security assistance, a presidential meeting or anything else," the deputy White House counsel Michael Purpura told the Senate on Saturday.

Bolton's reported claim also adds a significant data point to what Democrats have said is an "overwhelming" body of evidence against the president.

Trump, for his part, denied he ever told Bolton he would withhold Ukraine's aid until Zelensky gave in to his demands.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," the president tweeted. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

But it may not be so easy for Trump to write Bolton off.

The former national security adviser was not only a high-profile figure in Trump's inner circle, but a meticulous note-taker. Bolton is said to have created detailed documents of significant meetings and interactions — known as contemporaneous memos — and such notes are considered admissible evidence in a court of law.

It's unclear whether enough Republicans will budge on whether to call witnesses in Trump's trial in light of The Times' reporting. But it is clear that at the very least, this throws the Senate's proceedings into uncharted territory.

"The odds of deposition for new witnesses is certainly rising dramatically," one senior GOP official told The Washington Post after The Times' story broke.

"At this point, I think Democrats are going to focus on the need for Bolton's testimony and will not let go," Ohlin said. Republican Sen. Mitt Romney of Utah "will likely vote in favor of calling Bolton as a witness," he added, and "all eyes will shift" to Sens. Susan Collins, Lisa Murkowski, and Lamar Alexander, "all of whom will now be under immense pressure to vote for calling witnesses."

George Conway, a high-profile conservative lawyer married to the White House counselor Kellyanne Conway, wrote in a Post op-ed article that while it was already clear that Trump engaged in a quid pro quo with Ukraine — confirmed in testimony from Gordon Sondland, the US's ambassador to the European Union — Bolton's testimony on the matter "would make that devastating conclusion inescapable, even to Republican senators who have striven mightily to blind themselves to the obvious."

"From Trump on down, they all know how damning Bolton's testimony would be to Trump's defense," said Conway, who has frequently been critical of the president, adding: "If Bolton testifies to what's in his manuscript, these arguments, weak as they are, will collapse. The words will come from Trump's mouth, because Bolton will have put them there. The direct witness whose absence Trump's lawyers trumpeted will have appeared."