Transcript for Both sides debate amendments to impeachment trial rules
And good evening from Washington tonight. And we are here this evening because of the history being made in the senate chamber as we come on the air tonight. The impeachment trial of president trump now under way, in a deeply divided capital, a deeply divided nation. The chief justice of the supreme court, John Roberts, presiding. The first battle right from the start. A fierce argument over the rules laid out by senate majority leader Mitch Mcconnell. The Democrats calling it a coverup, arguing Mcconnell's framework does not allow evidence from the house investigation and gives no promise of any new witnesses. Democrats declaring the house did its job, the senate must now do theirs. The president's team, white house counsel pat sip Loney and his personal lawyer Jay skeulow accusing the Democrats of an investigation full of secrecy. Democrats saying the president blocked evidence and witnesses from the start. And what we noticed when I was allowed to sit in today. The key senators who were taking copious notes. ABC's Mary Bruce was with us all day here, and she leads us off tonight. The senate will convene as a court of impeachment. Reporter: With a bang of his gavel, chief justice John Roberts opening the senate impeachment trial of president trump. 100 senators sitting in silence. Their cell phones tucked into cubbies outside the chamber. Hear ye, hear ye, hear ye. All persons are commanded to keep silent. Reporter: But from the outset a fierce clash, as Republican leader Mitch Mcconnell proposed his rules governing the trial. It sets up a structure that's fair, even-handed and tracks closely with past precedents. Reporter: Mcconnell's plan would have compressed the trial, giving each side just two days to present their case, forcing marathon 12-hour sessions. He also wanted to exclude at the outset all the evidence Democrats gathered in the house investigation. A trial without evidence is not a trial. It's a coverup. Reporter: Some Republicans, like senator Susan Collins of Maine, weren't happy, either. They pushed back. And Mcconnell, eager to keep the key Republicans in his corner, quickly gave in. The changes scrawled by hand onto the text of the resolution. Mcconnell now giving three days of opening statements and allowing the existing evidence. But Democrats say that's not good enough. They want to be able to subpoena documents and call additional witnesses the white house has been blocking, including the president's former national security adviser John Bolton. The house calls John Bolton. The house calls Mick Mulvaney. Let's get this trial started, shall we? We are ready to present our case. We are ready to call our witnesses. The question is, will you let Reporter: Some Republicans have suggested they could do just that. Well, I think it's important to hear from John Bolton and perhaps other witnesses, obviously, from both the defense as well as the prosecution. The right time for that vote, that decision, is after the opening arguments. Reporter: Democrats urging senator Mitt Romney and his colleagues to take a stand. The house did its job. So now it's up to you. Reporter: But the president's lawyers are fighting back, they say house Democrats should have let the courts decide the question of witnesses. Obstruction? For going to court? It's an act of patriotism to defend the constitutional rights of the president. Reporter: Democrats introducing one resolution after another, trying to change the rules to allow them to subpoena documents. But their efforts failing. White house counsel pat Cipollone saying it's time for the two sides to make their case. It's long past time that we start this so we can end this ridiculous charade and go have an election. And so the trial under way tonight. Mary Bruce live inside the capitol, not far from where we are broadcasting tonight. Mary, as you know, reporters are allowed in. I sat in on the impeachment trial for a time today and I noticed so many of those key moderate Republicans you've reported on here, Susan Collins, Lisa murkowski, Mitt Romney, all taking notes. Mitt Romney had a notebook in hand. And they hold the power here when it comes to deciding whether they're going to join Democrats in asking for witnesses? Reporter: David, this trial and how long it lasts comes down to witnesses, and what four Republicans ultimately decide to do. Mitt Romney said he would like to hear from John Bolton. But for now, these Republicans are waiting to make a decision. First, they say they want to hear the arguments. And David, those could get under way tomorrow. Mary Bruce, we'll be here are you.
This transcript has been automatically generated and may not be 100% accurate.