Transcript for Cohen to accuse Trump of potential criminal conduct in office

the split screen, president trump's former lawyer and fixer, Michael Cohen, is about to publicly testify against him in Washington. Take a live look at the capitol. That is where Cohen will take the hot seat. Cecilia, what a dramatic day this will be. The last time congress heard testimony like this, more than four decades ago when president Nixon's lawyer John Dean implicated Nixon in the crimes of watergate. The testimony Cohen intends to give, a raw and personal broadside against the president's character and will implicate him in the crime sending Cohen to prison and Cohen will face direct attacks on his own character. Historic moment on capitol hill where Mary Bruce starts us off. Mary, the president may be thousands of miles away but he is focused on Michael Cohen. Reporter: And, George, this morning, the president is already trying to discredit Cohen's testimony. Tweeting that Cohen has just been disbarred for lying and fraud and that he is now, quote, lying in order to reduce his prison time. There is no question, this is a high-stakes do-over for Michael Cohen who has pleaded guilty to lying to congress before. But this morning, we've learned that Cohen will come armed with deep regret and a long list of damaging accusations against the president. This morning, Michael Cohen taking the oath and telling all, set to deliver explosive testimony about his former boss, president trump. At this point in time I really appreciate the opportunity that was given to me to clear the record and to tell the truth and I'm going to let the American people decide exactly who is telling the truth. Reporter: For the first time publicly, Cohen will accuse the president of criminal conduct while in office. The president's former lawyer and fixer intends to deliver a scathing opening statement calling trump a racist, a con man and a cheat. It's a dramatic reversal for the president's once fiercely lawyer I will do anything to protect Mr. Trump. Reporter: But now Cohen will allege as a presidential candidate trump knew his longtime adviser Roger stone was communicating with Wikileaks about hacked emails that would be damaging to Hillary Clinton and Cohen will suggest that trump implicitly told him to lie about a trump tower Moscow project, a deal Cohen admits trump's team was negotiating well into the campaign. Mr. Cohen, should the president be worried? What evidence do you have of criminal conduct? You lied before. Why should congress trust you now? Reporter: Cohen pleaded guilty to breaking campaign finance laws by making hush money payments to porn star stormy Daniels just 11 days before the election. Payments the president initially claimed to know nothing about but prosecutors say trump directed Cohen to arrange them and today Cohen will show the committee what he says is evidence. This $35,000 check he says president trump signed from his personal bank account as a partial reimbursement for that $130,000 payment. Do you have any to worry about on Michael Cohen's testimony? No. Reporter: With partisan tensions running high Republicans are on the attack. Congressman Matt Gaetz tweeting what seemed like a threat to reveal embarrassing personal information about Cohen saying Cohen's wife is about to learn a lot. Democrats are crying foul claiming Gaetz could have committed witness tampering but this morning Gaetz is apologizing tweeting it was not my intent to there'sen as some believe I did. I'm deleting the tweet and I should have chosen words that better showed my intent. I'm sorry. Now, in just a few hours Cohen will be grilled by the 42 members of the house oversight committee and while Cohen clearly has a long list of issues they wants to cover, members can also grill him on a wide range of topics and what he witnessed during his decade by the president's side. There is one issue, though, you're not likely to hear about in much detail, the Russia investigation because of the ongoing special counsel's probe, members of the committee have been told to steer clear of that issue, George. We'll see if they stick to that. Thanks very much. Epic day on capitol hill and let's talk about it with Dan Abrams and so much in the let's start out, this new charge where he says he was in the room when Roger stone told the president about Wikileaks' Julian assange. Let's separate out illegality from lies and embarrassing information. That's what you have to separate out as you watch the testimony here. This one falls into the if what Michael Cohen is saying is true, it is embarrassing, it shows that the president was not telling the truth. But not necessarily criminal. Because here you're just talking about contacts with Wikileaks, not with regard to the hack itself that the Russians did, not coordination with the Russians themselves but potentially getting advance information from Wikileaks about what they were doing with the information they had received from the Russian. What could be evidence of a crime, these checks. Let's put the check up on the screen. This check, August 1st, 2017, the president is in the oval office at this time, $35,000 check to Michael Cohen. They say this is evidence he was part of a conspiracy. Right, so here if they can show that the president knew that this was reimbursement for the payoff to stormy Daniels and that it was intended to influence the campaign and that remains the critical question there because the defense could be, yes, I knew that this was a payoff, but this was to protect my wife, this had nothing to do with the campaign, et cetera, but it is definitely dangerous business now that you have the signed check. The second check signed by don junior and the cfo of the trump organization. Right, so now you've got a separate question with regard to, for example, don junior. Is there a possible conspiracy charge with regard to violating campaign finance laws? Remember, he's not protected the way the president is from being indicted while in office. That's another avenue that we have to look at here, is are other people possibly going to get charged as a result of what Michael Cohen is saying. Remember the southern district prosecutors have said already it's a matter of fact that the president directed this. And let's be clear, the southern district of New York knows all of this. We're learning some of this and exactly how Michael Cohen knows this. The southern district of new York has all this information as does Mueller's team. So this isn't going to be new to them. The question for the public is now going to be to wonder what is the southern district going to do with the information that they have? Are there going to be potential charges now? That's what we got to look for. We hear Michael Cohen explain why he lied to the congress about trump tower Moscow. Well, he says, obviously, he lied for the president. That he was trying to cover up for him because they didn't want the public to know. But he does say in there that he was not instructed to lie to congress. He says that's not the way Donald Trump worked and he says it was actually more subtle than that. Dan Abrams, thanks very much. You will join our live coverage at 10:00 A.M. Cecilia, you'll be there too.

This transcript has been automatically generated and may not be 100% accurate.