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(CNN) —  

President Donald Trump’s ex-lawyer Michael Cohen will unleash a stunning indictment of his former client Wednesday that will amount to one of the strongest rebukes of a sitting President in living memory.

Cohen will accuse Trump in a congressional hearing of being a racist and a conman in testimony that will offer a rare televised glimpse into the cloud of scandals that have haunted Trump’s presidency.

According to testimony released ahead of his appearance, Cohen will say Trump knew that his former political adviser Roger Stone was aware of pending releases of stolen emails intended to damage Hillary Cinton’s presidential campaign.

The disclosure will revive speculation that the President colluded with representatives of Russia to interfere in the 2016 election, which evolved into an effort by Moscow to help Trump win in his duel against Clinton.

Cohen’s allegations, if true, would amount to the first time anyone with direct knowledge of the situation has said the President had advance knowledge of the WikiLeaks dump of Clinton emails.

Trump has said all along that there was “no collusion” between his campaign and Russia in 2016.

And the President provided an early rebuttal to Cohen’s testimony, tweeting from Hanoi just two hours before he is slated to meet Kim Jong Un one-on-one to kick off their second nuclear summit.

“Michael Cohen was one of many lawyers who represented me (unfortunately),” Trump tweeted. “He had other clients also. He was just disbarred by the State Supreme Court for lying & fraud. He did bad things unrelated to Trump. He is lying in order to reduce his prison time. Using Crooked’s lawyer!”

Cohen’s Wednesday appearance before the House Oversight Committee promises a look into the foundation of the concurrent scandals that have mostly been pursued by special counsel Robert Mueller, who has run an unusually leak-proof investigation by Washington standards.

Cohen’s quest for retribution has the potential to be a pivotal development in Trump’s term and will certainly be the most suspenseful public moment yet in the swirl of scandals afflicting the President.

RELATED: House panel to Dems: Russia off limits in Cohen’s public hearing

Adding to the intrigue, Trump will be watching 12 time zones away in the middle of the night in his hotel in communist Vietnam, the setting for his summit with North Korea’s tyrant leader.

Cohen, who served for years as Trump’s legal “fixer,” plans to offer behind-the-scenes details of Trump’s business practices and events that went on during his presidential campaign, a source told CNN’s Gloria Borger on Tuesday.

He may also provide documents to back up his claims, the source said, an important consideration since Trump’s Republican allies will seek to impugn Cohen’s credibility given that he has already pleaded guilty to lying to Congress, along with tax and fraud charges.

Tuesday’s hearing will likely raise a flurry of questions that will go far beyond Cohen’s trustworthiness as a witness. It could test Trump’s unmatched ability to slide past scandals that would doom any normal politician. And it may offer hints at the extent of the information Cohen has already given to prosecutors. The hearing could potentially also allow fresh assessments of steps, up to and possibly including an impeachment investigation, that Democrats could take against Trump in future.

Democratic Rep. Jackie Speier of California believes that Cohen’s life-changing decision to split with Trump and to take responsibility for his own financial crimes lends credibility to testimony that Cohen’s lawyer has promised will be “chilling.”

“I think he’s going to be pretty pure in terms of his responses, and I think there will be people who are implicated as a result of his testimony,” Speier, who will be one of the lawmakers questioning Cohen, told CNN’s Brianna Keilar on Tuesday.

White House press secretary Sarah Sanders preceded Trump in launching a pre-emptive strike against Cohen, saying he should not be believed in anything he says.