Third impeachment trial in US history begins Tuesday

The ongoing impeachment battle involving President Donald Trump is entering the Senate trial phase.
4:10 | 01/21/20

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Transcript for Third impeachment trial in US history begins Tuesday
tonight, new reporting on what we can expect, as the impeachment trial of president trump gets under way tomorrow. Each side, we've learned, will get 24 hours, two days each, to make their opening arguments. Then there will be 16 hours of questions from senators who took the oath to act as impartial jurors. After all of that, they will then debate whether to hear from witnesses. Tonight, president trump's legal team revealing how they plan to defend the president they say, quote, the president did absolutely nothing wrong. The house managers who will make the case against the president on the senate floor today. One of the president's lawyers tonight saying the articles of impeachment do not indicate the president committed any crime. The house Democrats meeting at the capitol in advance of making their case, as early as tomorrow, and ABC's Mary Bruce leads us off from the hill tonight. Reporter: President trump greeted with cheers and loud boos today as he paid tribute to Dr. Martin Luther king Jr. The president spending less than a minute at the memorial to the civil rights icon. Blocks away on capitol hill, Democrats are preparing to make their case against him, and tonight, the president's legal team is offering the first glimpse of their defense. In a 110-page brief, trump's lawyers say the process has "Violated every precedent and every principle of fairness" and are urging senators to swiftly reject the charges. One of the president's lawyers, Alan Dershowitz, laid out their central argument, that charges against trump don't rise to the level of "High crimes and misdemeanors" as the constitution requires. There has to be criminal behavior, criminal in nature. The articles of impeachment are two noncriminal actions, namely obstruction of congress and abuse of power. Reporter: But most constitutional experts disagree, and Dershowitz himself said the complete opposite 21 years ago during the Clinton impeachment. Certainly doesn't have to be a crime. If you have somebody who completely corrupts the office of president and who abuses trust and who poses great danger to our liberty, you don't need a technical crime. Reporter: For days, Dershowitz has been pressed on whether his client, the president, abused his power. Today, he still refused to give a straight answer. Do you personally believe Donald trumps right now, with the evidence you've seen in front of you, as one of his attorneys, abused his power, yes or no? I'm not going to answer that question yes or no. It's irrelevant. Reporter: Democrats argue "The facts are indisputable and the evidence is overwhelming." In their own brief, they outline the case for additional witnesses, saying, "Only if the senate sees and hears all relevant evidence, only if it insists upon the whole truth, can it render impartial justice." Make no mistake about it, we will force votes on witnesses and documents. Reporter: Even some Republicans, like Lisa murkowski of Alaska, say they're open to hearing additional witnesses, like former national security adviser John Bolton. I'm going to take my constitutional obligations very, very seriously. Let's get to Mary Bruce. And Mary, we mentioned off the top there we've learned tonight that each side will get 24 hours, two days each to make these opening statements. Then q&a from senators. And then this debate over whether to call witnesses. This is going to play out other several days. We know that four Republican senators will be watched very closely here. Will they join Democrats in asking for witnesses. Lisa murkowski, of course, Susan Collins of Maine, as well. Mitt Romney of Utah, Lamar Alexander of Tennessee. The big question, Mary, will they join Democrats who have been fighting for several days now for witnesses in this trial. Reporter: Well, David, these four key Republicans have suggested they want to hear from additional witnesses. Senator Mitt Romney said he would like to hear from John Bolton. The question is, after opening statements and those 16 hours of questioning, do those Republicans still feel that way? Do they want to hear from additional witnesses at that point? And then will they side with Democrats? David? Only the third time in American history. Mary Bruce leading us off tonight. Mary, thank you. And you and I will be here in Washington this time tomorrow, and of course joining George and our team. Live coverage begins tomorrow at 1:00 P.M. Eastern, right here on ABC.

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

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