Democrats have stepped up calls to allow witnesses in the Senate impeachment trial of Donald Trump after the publication of an account by his former top aide John Bolton that challenges the US president's defence.
Mr Bolton, the former national security adviser, reportedly alleges in a draft of a memoir that Mr Trump personally demanded a freeze on military assistance to Ukraine until it aided investigations into his Democratic political rivals, including former vice-president Joe Biden.
Details from Mr Bolton's as yet unpublished The Room Where It Happened: A White House Memoir were reported by the New York Times on Sunday night, just as Mr Trump's legal team was preparing to present its defence case in full this week.
"There can be no doubt now that Mr Bolton directly contradicts the heart of the president's defence and therefore must be called as a witness," the Democrats prosecuting the impeachment case said in a statement.
On the first day of the trial, when the senators determined the rules for the proceedings, the Democrats demanded the ability to call witnesses, but were rebuffed by the Republican majority. A few moderate Republicans, including Utah's Mitt Romney and Maine's Susan Collins, suggested they would be willing to support witnesses, but only in the second phase of the trial due to begin later this week.
Mr Bolton has been uncharacteristically reticent since he departed the White House in September after, according to Mr Trump, being fired by the president. However, he said in January that he was willing to testify in the president's Senate impeachment trial if subpoenaed.
Democrats are keen to hear from Mr Bolton because he has first-hand knowledge of some of the key meetings on Ukraine, but also because he left the White House following a rift with the president. This suggests he has less incentive to protect Mr Trump than some of the other officials they want to call, particularly Mick Mulvaney, White House chief of staff, who has refused to testify.
"Bolton regularly talked directly to Trump. He should have first-hand knowledge about what Trump directed others to do on his behalf," said Brendan Boyle, a Democratic congressman from Philadelphia. "His testimony is very important, which is exactly why the White House will do whatever it takes to block him from telling what he knows."
Even before details of Mr Bolton's memoirs leaked, Democrats' interest was piqued during the House's impeachment investigation, particularly by testimony given in November by Fiona Hill, his former top National Security Council Russia expert.
Ms Hill described how administration aides - including Mr Mulvaney and Gordon Sondland, US ambassador to the EU - helped carry out a pressure campaign against Ukrainian president Volodymyr Zelensky orchestrated by Rudy Giuliani, personal lawyer to Mr Trump.
Mr Giuliani wanted Ukraine to announce an investigation into Mr Biden, Mr Trump's potential 2020 election rival, and his son Hunter, who served on the board of a Ukrainian energy company. Mr Trump had made the same request in the July 25 2019 telephone call with Mr Zelensky that sparked the impeachment inquiry.
Ms Hill's most damaging testimony came when she recited comments by Mr Bolton, who had refused to testify before the House committees conducting the investigation, about elements of the Ukraine scandal.
Ms Hill described how Mr Bolton bristled in a July 2019 meeting when Mr Sondland told visiting Ukrainian officials that Mr Zelensky would not get a coveted Oval Office audience with Mr Trump until he announced the probe. She told lawmakers that Mr Bolton immediately ended the meeting and instructed her to tell the top National Security Council lawyer that he was "not part of whatever drug deal Mulvaney and Sondland are cooking up".
She also told lawmakers that Mr Bolton had at one point referred to Mr Giuliani as a "hand grenade who's going to blow everybody up" because of his efforts orchestrating the Ukraine pressure campaign.
Mr Bolton had been an unusual choice for national security adviser. His hawkish views - particularly his call for military action to induce regime change in Iran and North Korea - made him an unlikely foreign policy bedfellow for the president. Mr Trump had campaigned in 2016 on a much less muscular foreign policy than Mr Bolton would endorse.
"I actually temper John, which is pretty amazing," Mr Trump said last year about his aide.
Mr Trump eventually fell out with the Yale-educated lawyer in September, after rifts over Iran, North Korea and Turkey grew increasingly fraught. But Mr Bolton has yet to articulate his insights into the Ukraine affair in public.
Mr Bolton's claims put Senate Republicans in a difficult position. Over the course of four decades, the former national security adviser cultivated close ties to many leading Republican foreign policy hawks, a group that has bristled at Mr Trump's takeover of the party since his 2016 election.
If the Democrats are successful in convincing the Republicans - who have a 53-47 majority in the Senate - to allow witnesses, Mr Trump will he hoping that Mr Bolton leaves that reputation at the door of the Senate chamber.