Ex-FBI chief James Comey will tell Congress on Thursday President Donald Trump wanted a “patronage relationship” and asked for his “loyalty”.
According to his opening statement, Mr Comey will also testify the president asked him to drop an inquiry into fired National Security Adviser Mike Flynn.
He will say Mr Trump called the Russian probe “a cloud” over him.
Mr Comey also will back the president’s assertion that the FBI chief assured him Mr Trump was not under scrutiny.
Reacting to the prepared testimony on Wednesday evening, Mr Trump’s private legal counsel on the Russia inquiry, Marc Kasowitz, said the president was “pleased” Mr Comey had confirmed he was not in investigators’ crosshairs.
“The president feels completely and totally vindicated,” said the attorney.
In Thursday’s eagerly anticipated Senate Intelligence Committee hearing, Mr Comey will detail his interactions with Mr Trump leading up to his firing on 9 May.
It is one of several congressional panels that, along with the FBI, is investigating US intelligence assessments that Russian hackers meddled in last November’s presidential election in an effort to help Mr Trump beat Democratic candidate Hillary Clinton.
The inquiries are also investigating whether any Trump campaign officials colluded with the alleged Kremlin plot, which Moscow has repeatedly denied.
Mr Comey will say his first meeting with the president occurred on 6 January in a conference room at Trump Tower, where Mr Comey briefed him alone on “salacious and unverified” allegations about him.
A dossier compiled by a former British intelligence official had claimed the Russian security services possessed compromising material on Mr Trump, including that he had been recorded consorting with prostitutes at a Moscow hotel.
Mr Comey’s statement says the president “expressed his disgust for the allegations and strongly denied them” during a subsequent meeting.
That denial came in a one-on-one dinner on 27 January at the White House, Mr Comey will say, adding that he had a “very awkward conversation” with the president that evening.
Who do you believe? Anthony Zurcher, BBC News
It’s like a sneak preview of a blockbuster movie – this opening statement should be preceded with a “spoiler alert!” warning.
Mr Comey is going to largely confirm all the reports that were circulating in the media about what took place during his private meetings with the president – the talk of loyalty, the pressure on the FBI to ease off its Flynn investigation, the multiple requests for public confirmation that the FBI was not targeting Mr Trump himself.
While Mr Comey casts the president in a less-than-flattering light, he does shy away from directly accusing the president of obstruction of justice. Much will be made of how the two men may have interpreted the phrase “honest loyalty” differently. In addition, Mr Comey viewed the president’s Flynn request as only relating to his conversations with the Russian ambassador and not the investigation as a whole.
That may not matter much, however.
Large portions of the former director’s account is in direct contradiction with Mr Trump’s version of their meetings. It sets up a “he-said, he-said” situation – but Mr Comey has memos and conversations with other FBI officials to buttress his case.
What’s more, if recent polls are any indication, the American public trusts him more than the president. Mr Comey’s performance under questioning – particularly if Mr Trump responds angrily via Twitter – could further sour the public mood.
Mr Trump asked the FBI director during the discussion in the Green Room whether he wanted to stay in his job, Mr Comey will testify.
He will say he found this “strange” because Mr Trump had already told him twice in earlier conversations that he hoped he would not step down.
The former FBI director will testify the question “concerned me greatly” because he felt the dinner was an effort to “create some sort of patronage relationship”.
The former FBI director will say: “A few moments later, the president said, ‘I need loyalty, I expect loyalty.’
“I didn’t move, speak, or change my facial expression in any way during the awkward silence that followed. We simply looked at each other in silence.”
In testimony, the former FBI director will detail his next encounter with Mr Trump, during a meeting attended by intelligence chiefs at the White House on 14 February.
The president asked Mr Comey to stay at the end of the Oval Office meeting and told him: “I want to talk about Mike Flynn.”
Mr Trump accepted Mr Flynn’s resignation as national security adviser just 24 days into the job after he misled the White House about his conversations with the Russian ambassador.
Mr Comey will say Mr Trump told him: “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy.”
The former FBI director will testify that he offered no such assurance.
Mr Comey will also say the president phoned him on 30 March and said the Russia investigation was “a cloud that was impairing his ability to act on behalf of the country”.
The former FBI director will testify that Mr Trump “said he had nothing to do with Russia, had not been involved with hookers in Russia”.
Mr Comey will say he assured Mr Trump during their discussions on 6 January, 27 January and 30 March that the president himself was not under investigation.
He will testify that Mr Trump told him during the 30 March phone call: “We need to get that fact out.”
The former FBI director will say Mr Trump phoned him again on 11 April to press him on this matter. It was the last time they spoke.
Mr Comey will say he told the president the White House should contact the Department of Justice.
The former FBI director notes that he spoke with President Barack Obama only twice during the more than three years that their time in office overlapped.
But he can recall nine one-on-one conversations with Mr Trump in four months, three in person and six on the phone.
Mr Trump’s interactions with Mr Comey are likely to fuel Democratic suggestions that the president may have tried to obstruct justice.
However, the Republican president will seize on Mr Comey’s vindication of Mr Trump’s assertion that he himself was not under scrutiny.
Mr Trump said in his dismissal letter to Mr Comey: “I greatly appreciate you informing me, on three separate occasions, that I am not under investigation.”