Senate impeachment trial of Trump officially begins

The history-making moment included the swearing in of Chief Justice John Roberts.
4:59 | 01/17/20

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Transcript for Senate impeachment trial of Trump officially begins
Good evening and it's great to have you here you with on a Thursday night. And we begin tonight with the solemn turn on the hill today. The senate impeachment trial is here. And that heavy moment today when the chief justice arrived to be sworn in to preside over the impeachment trial of president trump. The managers from the house, the members of congress who will present the two articles of impeachment present their case, walking across the capitol to the senate. Then the formal reading of the two articles. Chief justice John Roberts presiding. Every senator standing with arm raised. It was silent but for that moment they promised impartial justice. Then, signing a pledge to do the same. And all of this tonight, amid a major new allegation involving Ukraine and the effort to get dirt on Joe Biden. This video tonight, the man standing next to the president there at ma a Lago, saying president trump knew exactly what was going on. President trump responding tonight, and we begin with Mary Bruce on this historic day. She's on the hill tonight. On the part of the house will be received -- Reporter: Today, on the senate floor, a solemn ceremony seen just twice before in American history. The trial of president Donald Trump, now officially under way. At 12:00 noon, the senate will receive the managers of the house of representatives to exhibit the articles of impeachment against Donald John trump, president of the united States. Reporter: A sight rarely seen. Nearly all 100 senators present, sitting quietly in their seats. Hear ye, hear ye, all persons are commanded to keep silent, on pain of imprisonment. Reporter: The chief justice John Roberts then sworn in. Raise your right hand. Do you solemnly swear that in all things pertaining to the trial of the impeachment of Donald John trump, president of the United States, now pending, you will do impartial justice according to the constitution and the laws, so help you god? I do. God bless you. Reporter:oberts then administering the oath to the full senate. Do you solemnly swear that in all things apertaining to the trial of the impeachment of Donald John trump, president of the United States, now pending, will you do impartial justice according to the constitution and the laws, so help you god? I do. Reporter: The senators then signing that pledge, one by one. Democratic leader chuck Schumer echoing its words with a warning to Republicans who've already vowed to stand by the president. Now that every senator has sworn an oath before god and the American people to do impartial justice, let every senator reflect on that choice and let history weigh on every one of our shoulders. Reporter: And even as the trial begins, explosive new allegations. An associate of the president's lawyer Rudy Giuliani now speaking out. I can't wait to come back. See you in Ukraine soon. Reporter: Lev parnas tells MSNBC he and Giuliani worked together to pressure Ukraine to investigate Joe Biden. And he says the president knew all about it. President trump knew exactly what was going on. He was aware of all my movements. He -- I wouldn't do anything without the consent of Rudy Giuliani or the president. Reporter: Parnas flatly dismissed the president's claim he only wanted Ukraine to investigate corruption. It was all about Joe Biden. It was never about corruption. Reporter: And parnas insists "Everyone was in the loop," including vice president Mike pence and attorney general bill Barr. Attorney general Barr was basically on the team. Reporter: Barr and pence deny it. And the white house points out that parnas is under federal indictment. Parnas says the president can try to distance himself but tonight, his lawyer releasing this video of trump and parnas together at mar-a-lago in 2017. And Democrats say parnas' revelations prove their point that the senate should allow more evidence and more witnesses to be heard. The very integrity of the United States senate is on trial. Let's get to Mary Bruce on the hill again tonight. And all of this as Republicans have said the president committed no crime here. But tonight, yet another development. Now a nonpartisan federal watchdog agency has determined the president, in their words, did break the law when he withheld nearly $400 million in U.S. Military aid for Ukraine, it had been passed by congress. The government accountability office saying tonight, quote, faft vl execution of the law does not permit the president to substitute his own policy priorities for those that congress has enacted into law. And Mary, tonight, Democrats are now seizing on those words? Reporter: David, the speaker says the watchdog's conclusion further strengthens the house's case for impeachment and remoal and reinforces the need for a fair trial in the senate. The administration tonight, not surprisingly, says they disagree with the opinion that the president broke the law. David? All right, Mary Bruce leading us off and there on the hill all day today for us. As that somber ritual was

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

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