A fresh leak from John Bolton's book came just hours before the Senate is expected to vote against calling witnesses in President Trump's impeachment trial.

Drafts of Bolton's book say Trump instructed his then-national security adviser to get involved in pressuring Ukraine to obtain damaging information about the president's political rivals, as reported by the New York Times on Friday.

The directive was made during an Oval Office meeting in May, attended by White House chief of staff Mick Mulvaney, White House counsel Pat Cipollone (who is now leading Trump's impeachment defense), and Trump's lawyer Rudy Giuliani, two months before Trump pressed Ukrainian President Volodymyr Zelensky during a phone call to help collect information that would be politically advantageous to him. A whistleblower complaint about this phone call gave rise to impeachment proceedings.

Bolton wrote that Trump ordered him to call Zelensky and urge the Ukrainian leader to meet with Giuliani, who was working to collect damaging information on Democrats such as former Vice President and 2020 presidential candidate Joe Biden and his son Hunter Biden, who was employed by a Ukrainian gas company, in the name of fighting corruption. Bolton claimed he never made the call.

Giuliani denied the Oval Office conversation as described in Bolton's manuscript ever took place. “It is absolutely, categorically untrue,” he told the New York Times.

The disclosure is certain to influence senators' thinking as they prepare to vote on whether to call for witnesses and documents on Friday. Democrats who levied two Ukraine-related articles of impeachment against Trump have used the leaks from Bolton's book draft to push centrist Republican senators to vote in favor of summoning Bolton and other Trump administration officials to testify during the Senate trial.

Democrats have wanted Bolton to testify ever since his former aide, Fiona Hill, testified to House impeachment investigators that Bolton opposed the Ukraine pressure campaign. She said Bolton wanted no part of what he described as a "drug deal" that fellow Trump administration officials were "cooking up."

Further raising the stakes was House Foreign Affairs Chairman Eliot Engel, a New York Democrat, revealing on Wednesday that Bolton contacted him in September "unprompted" urging him to look into the recall of U.S. Ambassador to Ukraine Marie Yovanovitch, whom Giuliani and a Ukrainian prosecutor accused of being a political enemy of the White House. Engel and other Democratic leaders began the impeachment investigation one day after that discussion.

Last night, a key Republican senator, Lamar Alexander of Tennessee, announced he would not vote in favor of calling additional witnesses to testify, making it more likely the trial could be over by this weekend. With a handful of centrist Republicans expressing interest in calling witnesses, GOP leaders are trying to avoid a tie vote. While a motion to call witnesses would fail in a tie, Supreme Court Chief Justice John Roberts, who is presiding over the trial, could intercede to break the tie, although most senators believe that is unlikely.

The leaks from Bolton's manuscript have prompted speculation about who is behind them. Trump's allies pointed to the National Security Council, where Bolton's book draft had been submitted for a standard review in December. Bolton, who left the White House in September, has said he would testify if subpoenaed, but Trump has threatened to block his testimony.