Pat Cipollone, the president's lead attorney in the impeachment trial, could also be a witness, John Bolton's unpublished book says, according to The New York Times.

The White House counsel was reportedly at a meeting in which President Donald Trump asked Bolton to help with his effort to pressure Ukraine into helping him dig up dirt on his political rivals.

The report raises ethical questions, and House impeachment managers have demanded that Cipollone disclose any firsthand evidence.

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The lead attorney in President Donald Trump's Senate impeachment trial has firsthand knowledge of the effort to pressure Ukraine into digging up dirt on Trump's political rivals, the former national security adviser John Bolton's forthcoming book says, according to The New York Times.

The Times' report, which cites Bolton's unpublished manuscript, raised the possibility that White House counsel Pat Cipollone, who has been leading Trump's defense team, could also be a potential witness.

The manuscript said Trump directly asked Bolton to help with the effort to pressure Ukraine in a meeting more than two months before the president's July 25 phone call with Ukrainian President Volodymyr Zelensky. The call sparked a whistleblower complaint that spiraled into an impeachment inquiry and ultimately led Trump to become the third president in US history to be impeached.

During this meeting, Trump reportedly asked Bolton to call Zelensky to ensure he would meet with the president's personal lawyer Rudy Giuliani, who has been at the center of the impeachment saga and engineered the campaign to urge Ukraine to launch investigations. Bolton did not make the call, the manuscript said, according to The Times.

Cipollone was at this meeting, the manuscript said, suggesting he witnessed Trump order a top adviser to get involved in the coordinated effort to pressure Ukraine. This raises ethical questions about whether it was appropriate for Cipollone to lead Trump's defense in a trial pertaining to matters he's said to have firsthand knowledge of.

Giuliani and Mick Mulvaney, the acting White House chief of staff, were also reportedly at the meeting.

The White House did not immediately respond to Insider's request for comment.

Trump and Giuliani both denied that the meeting happened when questioned by The Times.

"I never instructed John Bolton to set up a meeting for Rudy Giuliani, one of the greatest corruption fighters in America and by far the greatest mayor in the history of NYC, to meet with President Zelensky," Trump said. "That meeting never happened."

Giuliani told The Times that Cipollone and Mulvaney were never involved in meetings on Ukraine, though impeachment witnesses have explicitly linked Mulvaney to the efforts to pressure Kyiv.

"It is absolutely, categorically untrue," Giuliani said.

'A material witness to the charges against President Trump'

House impeachment managers have demanded that Cipollone disclose any firsthand evidence. "You may be a material witness to the charges against President Trump, even though you are also his advocate," they wrote in a letter to Cipollone, Politico reported.

The Times reported over the weekend that Bolton said in his book that there was an explicit quid pro quo linking roughly $400 million in military aid to Ukraine to the investigations Trump wanted. Friday's report added to this, amid congressional Democrats' mounting calls for Bolton to be called as a witness in Trump's impeachment trial.

Republicans have pushed back on calling witnesses and as of Friday afternoon appeared poised to block Democratic efforts to include them in the trial.

On Friday, another Republican who was considered to be on the fence about witnesses — Sen. Lisa Murkowski — announced that she planned to vote against calling any.

"Given the partisan nature of this impeachment from the very beginning and throughout, I have come to the conclusion that there will be no fair trial in the Senate," the Alaska Republican said in a statement. "I don't believe the continuation of this process will change anything."

She added, "It is sad for me to admit that, as an institution, the Congress has failed."

All 15 Senate impeachment trials in US history, including those for the two other presidents who were impeached, have called witnesses.

In the July 25 call, Trump urged Zelensky to launch inquiries into former Vice President Joe Biden and his son Hunter Biden and into a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US election. The request for an inquiry into the Bidens was linked to Hunter Biden's work for a Ukrainian natural-gas company, Burisma Holdings. There's no evidence of wrongdoing by either of the Bidens.

Trump's defense team has focused on Hunter Biden and Burisma throughout the trial, pushing out a slew of disinformation in the process.

Schiff: 'Yet another reason why we ought to hear from witnesses'

Democratic Rep. Adam Schiff, the lead House impeachment manager, zeroed in on The Times' report and Cipollone as the trial resumed on Friday, reiterating his calls for witnesses.

Schiff quoted The Times' report: "More than two months before he asked Ukraine's president to investigate his political opponents, President Trump directed John R. Bolton, then his national security adviser, to help with his pressure campaign to extract damaging information on Democrats from Ukrainian officials, according to an unpublished manuscript by Mr. Bolton. Mr. Trump gave the instruction, Mr. Bolton wrote, during an Oval Office conversation in early May that included the acting White House chief of staff, Mick Mulvaney, the president's personal lawyer Rudolph W. Giuliani and the White House counsel, Pat A. Cipollone, who is now leading the president's impeachment defense."

"You will recall Mr. Cipollone suggesting that the House managers were concealing facts from this body. He said, 'All the facts should come out,'" Schiff added. "Well, there's a new fact, which indicates that Mr. Cipollone was among those who were in the loop. Yet another reason why we ought to hear from witnesses."