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Democrats have intensified their calls for former US National Security Adviser John Bolton to testify in President Donald Trump's impeachment trial.

The pressure has grown following the report of a potentially explosive claim Bolton is said to make in a new book.

The New York Times cited a leaked Bolton manuscript as saying that Trump told him he wanted to freeze aid to Ukraine until Kyiv helped with investigations against the Democrats, including former Vice-President Joe Biden.

The US president rejected the report, which could undercut his denials that he had corrupt motives in asking Ukrainian President Volodomyr Zelensky in a phone call last July to investigate Biden, who is Trump's potential White House rival.

Bolton, a Republican, is an unlikely hero for Democrats. Still, they believe he will act as a star witness, one who will provide irrefutable evidence of wrongdoing by Trump and help bolster the case for his removal from office.

Democrats once invested similar hopes in another Washington figure, Robert Mueller, the former special counsel.

But Mueller looked miserable while testifying in May about his Russian investigation, and did not change many people's minds about the president.

Bolton could be different - or so Democrats hope.

Yale-educated Bolton, 71, was national security adviser from 2018-19.

He was "personally involved" in the president's dealings with the Ukrainian officials, according to Bolton's lawyer.

The former national security adviser was "at the nerve centre for all important decisions", says Matthew Spence, a former deputy assistant secretary of defence.

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If Bolton were to testify, he would be able to provide the most detailed account to date of the president's alleged political pressure on President Zelensky and the decision to freeze nearly $400m (£306m) in security aid.

Other witnesses have alleged that Trump administration officials used the aid as a bargaining chip to prod the Ukrainians into investigating the Bidens.

But they have not linked Trump explicitly to the withholding of aid in exchange for an investigation, or shown that Trump personally directed the operations.

Democrats believe the former national security adviser could provide the smoking gun.

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"Bolton was present during the crime," says Evelyn Farkas, who served as top Russia official during the Obama administration and is now running as a Democrat for a congressional seat in New York.

"And he knew that it was a crime at the time."

According to reports, Bolton opposed the withholding of security aid to Ukraine, and tried unsuccessfully to convince the president to release the military aid during an Oval Office meeting.

"This is in America's interest," the former national security adviser told the president, according to the New York Times, as he argued the aid should be provided to Ukraine.

The aid was eventually released - a day after Bolton acrimoniously left the White House.

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Trump has repeatedly said that he did nothing wrong and that the impeachment proceedings are a "scam".

White House lawyers have already turned down requests from Democratic lawmakers for Bolton and other witnesses to testify.

Trump's lawyers say their testimony would violate the president's right to confidentiality.

What's in the book?

Trump told Bolton the $391m in security aid to Ukraine would be frozen until investigations into the Bidens were opened

Acting White House chief of staff, Mick Mulvaney, was present for a phone call between the president and his lawyer Rudy Giuliani as they talked about the ambassador

Secretary of State Mike Pompeo said claims by Giuliani about the ambassador were false

Attorney General Bill Barr was notified of Bolton's concerns about the July call - something the Justice Department has denied

Bolton has said publicly he would testify if he gets a legal summons, however, ignoring the wishes of the White House lawyers who want him to lay low.

Senators will vote on the matter of witnesses in the coming days.

Bolton certainly knows a lot.

Whenever I saw him, whether in meetings overseas or in the West Wing, he always had a notebook.

A long-time commentator on Fox News, he jotted down his thoughts on paper while he was working in the White House.

Presumably the notes came in handy for his reported multi-million dollar book deal with Simon & Schuster.

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"Bolton will be a powerful witness for Democrats," says Jeremy Shapiro, who served for years as a career diplomat in the US state department.

"He's an assiduous note taker and was always keen to write things down. He's going to have the extra credibility that comes from that."

But some observers of the White House wonder if Bolton knows as much as he appears to, or if his willingness to testify is just a ploy to sell his book, The Room Where It Happened: A White House Memoir, which is out next month.

Bolton championed hardline policies towards North Korea, Afghanistan and Iran, and the president did not always agree with him.

At times Bolton was sidelined. Democrats also question whether the testimony from Bolton, regardless of its firepower, will make a difference.

Bolton is likely to be a more compelling speaker than Mueller, who suffered visibly during his presentation.

In contrast, Bolton loves the spotlight: he used to bounce on the balls of his feet when he stood at a podium in the West Wing. "He's going to be better television than Mueller," says Shapiro.

But Shapiro and others wonder whether Bolton will have much of an impact. "It's abundantly clear that the president is guilty as charged, and it's hard to improve on a slam dunk," he says. "It's also abundantly clear that the Republican senators don't care."

They would never convict this guy of anything, he adds.

The trial so far has largely followed Senate Republican leader Mitch McConnell's script.

But while it appears to be heading towards an acquittal of the president, Democrats will hope the Bolton book serves up a plot twist.