US impeachment: Memo confirms Donald Trump urged Ukraine to investigate rival Joe Biden

In the call, Mr Trump raised allegations, without citing any evidence, that the former vice-president sought to interfere with a Ukrainian prosecutor's investigation of his son Hunter.

"There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that," Mr Trump said to Ukrainian president Volodymyr Zelenskiy.

The conversation between the two leaders is one piece of a whistleblower's complaint, which followed the 25 July call.

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The White House account of the call reveals Mr Trump was willing to engage a foreign leader to dig up dirt on a political foe and he goes so far as to volunteer his attorney general to help.

But Mr Trump appears to stop short in the call of any explicit quid pro quo, such as linking Ukraine's help to American military aid or other assistance.

Days before the call, Mr Trump froze nearly $400 million (£323m) in aid to Ukraine.

It was not clear from the summary whether Mr Zelenskiy was aware of that. The president has insisted he did nothing wrong and has denied that any request for help was tied to the aid freeze.

It is illegal under federal law to seek foreign government assistance for US elections.

The release of the rough transcript today sets the parameters of the political debate to come.

Mr Trump, at the UN today, dismissed it and said as he often does that he is the victim of "the single greatest witch hunt in American history."

Democrats say it lays the groundwork for the congressional impeachment inquiry.

Mr Trump aides believed that his oblique, message-by-suggestion style of speaking would not lend itself to the discovery of a "smoking gun" in Wednesday's summary. His previous messages to his staff were at the centre of special counsel Robert Mueller's investigation into obstruction of justice in the Trump-Russia case.

In the conversation, Mr Trump doesn't distinguish between the roles of Mr Giuliani, his personal attorney and political ally, and MR Barr, who as the nation's top law enforcement officer is supposed to be above the political fray.

Mr Barr has been a staunch defender of Mr Trump, most notably during the Mueller investigation.

"Mr Giuliani is a highly respected man, he was the mayor of New York City, a great mayor, and I would like him to call you," Mr Trump said, according to the call summary. "I will ask him to call you along with the attorney general."

Immediately after saying Mr Giuliani and Mr Barr would be in touch, Mr Trump references Ukraine's economy, saying: "Your economy is going to get better and better I predict. You have a lot of assets. It's a great country."

It is not the first time Mr Trump has sought foreign assistance to undermine a political rival. He publicly asked Russia to find missing Hillary Clinton emails in 2016, but this is his first documented time doing so while president with the weight of the US government at his disposal.

The president took the 30-minute call from the White House residence, while officials in the Situation Room listened in and worked to keep a record of the conversation, as is standard practice. They used voice recognition software, but the call was not recorded. Mr Trump ordered the document declassified Tuesday.

The release came against the backdrop of the president presiding over a meeting of world leaders at the United Nations, a remarkable split screen even for the turbulence of the Trump era.

The inspector general for the intelligence community wrote to the acting Director of National Intelligence in August that he believed the conversation between Mr Trump and Ukraine's leader could have been a federal campaign finance violation because the president could have been soliciting a campaign contribution from a foreign government, a Justice Department official said.

The whistleblower - a member of the intelligence community - said in their complaint that they had heard the information from "White House officials", but did not have first-hand knowledge of the call, the Justice Department official said.

Prosecutors from the department reviewed a transcript of the call and determined the president did not violate campaign finance law. The determination was made based on the elements of the allegation, and there was no consideration of the department's policy that a sitting president cannot be indicted, the official said.

Justice Department spokeswoman Kerri Kupec said the attorney general was first notified of Mr Trump's conversation with the Ukrainian president "several weeks after the call took place", when the department received the referral about potential criminal conduct.

"The president has not spoken with the attorney general about having Ukraine investigate anything relating to former vice-president Biden or his son. The president has not asked the attorney general to contact Ukraine - on this or any other matter," the spokeswoman said.

Mr Trump has sought to implicate Mr Biden and his son in the kind of corruption that has long plagued Ukraine.

Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv. Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice-president or his son.

Lawmakers have been demanding details of the whistleblower's complaint, but the acting director of national intelligence, Joseph Maguire, has refused to share that information, citing presidential privilege.

He is to testify tomorrow before the House, and lawmakers are expected to have access to details of the complaint beforehand in a classified setting.

The complaint has set off a stunning turn of American political events, leading Ms Pelosi to yield to mounting pressure from fellow Democrats on the impeachment inquiry.

Mr Trump, who thrives on combat, has all but dared Democrats to take this step, confident the spectre of impeachment led by the opposition party will bolster rather than diminish his political support.