US President Donald Trump on Wednesday slammed John Bolton, a potentially damaging witness at his Senate impeachment trial, amid White House efforts to block publication of an explosive book by the former national security advisor.

The 100 members of the Senate have been sitting silently for six days listening to impassioned arguments for and against Trump’s removal from office for abuse of power and obstruction of Congress.

On being asked whether the ex-advisor should appear, chief Democratic prosecutor Adam Schiff said that his testimony was essential to conduct a “fair trial.”

“Don’t wait for the book,” the California lawmaker told the senators sitting as trial jurors. “This case is overwhelmingly clear without John Bolton but if you have any question about it you can erase all doubt.

“When you have a witness who is as plainly relevant as John Bolton, who goes to the heart of the most serious and egregious of the president’s misconduct, who has volunteered to come and testify — to turn him away, to look the other way, I think is deeply at odds with being an impartial juror,” Schiff said.

White House deputy counsel Patrick Philbin countered that the Democratic-controlled House of Representatives, which impeached the Republican president on December 18, had conducted their inquiry in a “half-baked partisan fashion” and had not issued their own subpoena for Bolton’s testimony.

Democrats have been pressing for the Senate to subpoena Bolton, who was fired by Trump in September, after reports that his upcoming White House tell-all book corroborates the abuse-of-power impeachment charge against the president over his dealings with Ukraine.

On January 7, Bolton had said that he is willing to testify in the expected Senate impeachment trial of the president, a surprise development that could complicate a weeks-long dispute over how the trial would play out.

Earlier, Bolton was so alarmed by a White House–linked effort to pressure Ukraine to investigate Democrats, he told aide Fiona Hill to alert the National Security Council’s chief lawyer.

He also warned that President Donald Trump’s personal attorney Rudy Giuliani, who has emerged as the point man in the president’s alleged drive to pressure Ukraine to dig up dirt on Democratic presidential hopeful Joe Biden, is “a hand grenade who’s going to blow everyone up,” his aide Fiona Hill told lawmakers, New York Times reported.

Ukraine’s former president had said that he discussed investments with President Donald Trump’s attorney, Rudy Giuliani, in 2017, but that he never discussed Ukrainian companies with any US official.

On Tuesday, the president’s lawyers urged the US Senate while closing their impeachment defense by accusing the Democrats of placing him on trial for political reasons.

Last week, Schiff called dramatically for the Senate to remove Trump from office and said that the US leader cannot be trusted to put the country’s interests ahead of his own.

His impassioned words capped a long day in which Democrats detailed Trump’s illicit scheme to pressure Ukraine to help his 2020 reelection campaign.

Earlier this week, the second phase of Trump’s impeachment trial began in Senate house Republicans determined to acquit him- and quickly, if possible.

Democrats, who control the House of Representatives and led the investigation, accuse Trump of manipulating Ukraine by withholding nearly $400 million in military aid for its war against Russian-backed separatists and a White House meeting for Zelensky until the latter announced a Biden probe.

On December 18, President Trump was formally impeached in a historic vote in the House of Representatives.

Trump’s legal team presented its line of defence for his impeachment probe, a process they dismissed as unconstitutional and “dangerous”.

In September, the impeachment inquiry, which Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.

Democrats are hoping their arguments will at least persuade some Republicans, who hold a 53-47 majority in the Senate, to support their call to issue subpoenas next week for four top current and former Trump aides to testify, and for internal White House records about the Ukraine affair.

(With inputs from agency)