The unpublished manuscript of President Donald Trump’s former national security advisor John Bolton states that Trump explicitly tied withholding $391 million in American security aid to Ukraine officials announcing specific investigations, according to a New York Times account.

Trump’s attorneys have defended the president in the impeachment trial currently before the Senate by claiming that the aid to Ukraine was not tied to investigations into former Vice President Joseph R. Biden’s son Hunter and his connection to Ukrainian energy firm Burisma, but Bolton’s account undermines that defense.

Early Monday morning, Trump tweeted: “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. 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.”

Bolton left the Trump White House in early September. He reportedly circulated drafts of the manuscript for his book, set to be published in March, to close associates, and sent a draft to the White House for the standard review process. The New York Times reports that they spoke to multiple people about Bolton’s manuscript.

Democrats want to call first-hand witnesses like Bolton and White House Chief of Staff Mick Mulvaney to testify in the Senate impeachment trial, and this revelation significantly bolsters the chance that four Senate Republicans will break ranks and vote in favor of witnesses appearing at the trial.

“I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton,” said Republican Sen. Mitt Romney on Monday.

According to the New York Times’ sources, Bolton brought up concerns over the Ukraine aid delay in August, and Trump responded that he “preferred sending no assistance to Ukraine until officials had turned over all materials they had about the Russia investigation related to” Biden, supporters of Hillary Clinton in Ukraine, and the Democratic National Committee’s  hacked server.

Secretary of State Mike Pompeo and Defense Secretary Mark T. Esper backed Bolton up and pressed Trump to release the aid, according to Bolton. Bolton also says that Pompeo knew Giuliani’s claims against then-American ambassador to Ukraine Marie L. Yovanovitch “had no basis,” reports the New York Times, and that Pompeo suspected the President’s personal lawyer wanted Yovanovitch out because she may have been interfering with the business interests of Giuliani’s other clients.

Despite his suspicions, Pompeo was ultimately unsuccessful at convincing Trump and went along with his order to remove Yovanovitch in May. On Friday, Pompeo cursed at an NPR reporter who questioned him about his role in removing Yovanovitch.

Bolton writes that he raised his concerns about Giuliani’s meddling to Attorney General William Barr after the July 26 call with Ukraine’s president. Barr denied that account through a spokeswoman, according to the paper.

But Fiona Hill, Trump’s former White House advisor on Russia and Europe, testified before the House impeachment inquiry in October that Bolton told her to notify White House lawyers about the interference she was experiencing from Trump’s ambassador to the European Union Gordon Sondland and other officials who claimed to be doing the President’s bidding on Ukraine policy.

Hill told the House that Bolton said he wanted no part of “whatever drug deal” was being cooked up to pressure Ukraine. “Giuliani’s a hand grenade who’s going to blow everybody up,” he told Fiona Hill.

The details included in Bolton’s manuscript are not new. We have heard all this from a bevy of witnesses in the House impeachment trial, including Sondland, who testified that “there was a quid pro quo” regarding U.S. policy towards the Ukraine and that top Trump officials Secretary of State Mike Pompeo, former National Security Adviser John Bolton, Vice President Mike Pence, and President Donald Trump all “knew what we were doing and why;” “everyone was in the loop. It was no secret.”

But Bolton’s account is extremely significant because the reports “strengthen the case for witnesses” in the Senate impeachment trial, said Sen. Susan Collins, another key swing vote, in a Monday statement. They “have prompted a number of conversations with my colleagues.”

During the House impeachment inquiry, Bolton contested the subpoena he was served on the grounds of executive privilege, and the House decided to drop the subpoena instead of pursue it through the courts. But Bolton now says that if he is subpoenaed by the Senate he will appear, reports the Times.

Trump and his attorneys do not want Bolton or other witnesses or documents in the trial.

“I think the timing of all of this is very, very, suspect” said White House press secretary Stephanie Grisham said in a Fox News interview Monday. “We stand by exactly what we’ve been saying all along.”

In a statement Sunday, Bolton’s attorney blamed the manuscript disclosure on corruption in the prepublication review process.

“It is clear, regrettably, from The New York Times article published today that the prepublication review process has been corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript,” said the statement.