US President Donald Trump's ex-lawyer Michael Cohen is expected to claim Mr Trump knew beforehand about a leak of hacked Democratic emails.
In congressional testimony, Cohen will also say Mr Trump directed plans for a Moscow skyscraper even during his White House campaign.
According to prepared testimony, Cohen will brand Mr Trump a "racist", a "conman" and a "cheat".
Mr Trump hit back: "He is lying in order to reduce his prison time."
The US president took time out from preparing to meet North Korean leader Kim Jong-un in Hanoi, Vietnam, on Wednesday to lash out in a tweet at his former fixer.
Cohen, 52, will start a three-year prison term in May for the campaign finance violation of paying hush money to one of Mr Trump's alleged mistresses, tax evasion and lying to Congress.
What will Cohen say about the email leak?
In prepared comments for his public testimony to the House of Representatives Oversight Committee on Wednesday, Cohen is expected to say he was in Mr Trump's office in July 2016 when Roger Stone, a longtime political adviser, called the then-Republican presidential candidate.
US media have published a copy of his opening statement online.
Cohen will say Mr Stone told Mr Trump he had been speaking to Wikileaks founder Julian Assange, who told him there would be a leak of emails within a couple of days that would politically embarrass Hillary Clinton's White House campaign.
Mr Trump has denied having prior knowledge of Wikileaks' disclosure of Democratic National Committee (DNC) emails during the election.
The messages caused a damaging rift among liberals by exposing Democratic officials' favouritism towards Mrs Clinton over her challenger for the party's White House nomination, Bernie Sanders.
Mr Stone, a self-proclaimed political dirty trickster, is currently facing charges of lying to Congress about his communications with Wikileaks and witness tampering.
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What will Cohen say about Moscow project?
He will say Mr Trump repeatedly inquired about plans for a Trump Tower Moscow while stating publicly that he had no business dealings in Russia.
The former attorney will say: "He lied about it because he never expected to win the election. He also lied about it because he stood to make hundreds of millions of dollars."
However, Cohen told Congress in 2017 that attempts to build a Trump skyscraper in Moscow had stopped by January 2016.
He has since admitted negotiations actually continued until June 2016 in the midst of the election campaign, though the real estate project ultimately did not go ahead.
Cohen is expected to apologise on Wednesday for his earlier false statements to Congress, which he will claim were "reviewed and edited" by Mr Trump's lawyers.
What else will he say about Russia?
Cohen is also expected to testify that, contrary to Mr Trump's repeated claims, he had advance knowledge of a meeting at Trump Tower in Manhattan between his campaign aides and a Russian lawyer promising "dirt" on Mrs Clinton.
The June 2016 meeting has been investigated by special counsel Robert Mueller, who is winding up a 21-month justice department inquiry into whether the Trump campaign colluded with an alleged Kremlin plot to influence the 2016 US presidential election.
Cohen is expected to relate an incident when Mr Trump's son, Donald Jr, walked behind his father's desk and told him in a low voice: "The meeting is all set."
Mr Trump, Cohen is set to testify, replied: "Ok good, let me know."
Cohen will also testify that he has no direct evidence that Mr Trump or his campaign colluded with Russia.
"I do not," his prepared testimony says. "I want to be clear. But I have my suspicions."
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What about the racism allegations?
Cohen is expected to tell lawmakers that Mr Trump is a racist.
According to his prepared testimony, Cohen will say: "He once asked me if I could name a country run by a black person that wasn't a 'shithole.'
"This was when Barack Obama was President of the United States.
"While we were once driving through a struggling neighbourhood in Chicago, he commented that only black people could live that way.
"And, he told me that black people would never vote for him because they were too stupid. And yet I continued to work for him."
What else will Cohen say?
He is expected to tell lawmakers that he wrote letters on behalf of Mr Trump threatening the Republican candidate's high school, colleges and the College Board not to release his grades to reporters.
Cohen reportedly plans to provide evidence of reimbursements he received from the president for hush money Cohen has admitted paying to a porn star who says she had an affair with Mr Trump.
Cohen is expected to show the committee a copy of a $35,000 (£26,000) cheque that Mr Trump signed in August 2017.
He will say it was one in a series to reimburse him for the $130,000 payment to Stormy Daniels.
"Lying to the First Lady is one of my biggest regrets," Cohen is expected to say. "She is a kind, good person. I respect her greatly – and she did not deserve that."
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How are Republicans responding?
North Carolina Republican Mark Meadows, who chairs the conservative House Freedom Caucus, has expressed outrage that Democrats invited a "convicted liar" to testify.
In an interview with the BBC before the hearing began, the Trump ally said: "I don't know that we've ever had a witness come before our committee with less credibility than Michael Cohen."
Florida Republican lawmaker Matt Gaetz, a fierce defender of Mr Trump, has meanwhile deleted a tweet he posted threatening to reveal compromising information about Cohen's personal life.
Mr Gaetz denied the tweet amounted to witness-tampering, contending it was "witness-testing".
Cohen's originally scheduled testimony was postponed earlier this month, after he cited "threats against his family" brought by Mr Trump and his lawyer Rudy Giuliani.