A look at the Republicans who might break with the party to vote for new witnesses.

Credit…Calla Kessler/The New York Times

Republican moderates are in the spotlight on Friday as House managers conclude their oral arguments and senators turn to the question of whether to call witnesses and seek new documents in the impeachment trial. All four of the senators opposed Democratic motions for witnesses and documents at the beginning of the trial, but have said they might be open to switching their stances after opening arguments have been completed.

So far, however, none have committed to do so.

Here are the Republican senators to watch:

Mitt Romney of Utah has not said much since the trial started. But earlier, he indicated he would be open to new witnesses, and said he wants to hear from John R. Bolton, the president’s former national security adviser.

Susan Collins of Maine is usually a swing vote in the Senate. Facing re-election this year, she is facing brutal blowback in her state for voting to confirm Justice Brett Kavanaugh for his seat on the Supreme Court. She has strongly suggested that she will ultimately vote to call witnesses. Doing so could help her mend fences with moderate voters she needs to keep her seat.

Lisa Murkowski of Alaska is an independent voice in the Senate. She was the only Republican to oppose Justice Kavanaugh’s confirmation and has indicated she could be open to having the Senate examine additional evidence in the impeachment case.

Lamar Alexander of Tennessee is retiring after a long career in the Senate. He has not given clear answers to whether he might support additional witnesses and is extremely close with Senator Mitch McConnell, the Republican leader. But Democrats hope his institutionalist impulses might prompt him to be the fourth vote they need.

There has been additional focus on a fifth senator, Cory Gardner of Colorado. Mr. Gardner is a first-term senator who is facing a tough re-election race this year in a politically competitive state. He will need support from independent voters and even some Democrats to win, but Mr. Garnder has so far been mum on the question of witnesses, and has criticized the impeachment inquiry as a politically motivated exercise.

Credit…Calla Kessler/The New York Times

Even as the Senate geared up for the third day hearing from prosecutors in the impeachment trial, a different kind of political clash was gathering outside the Capitol.

People attending the annual March for Life — and counterprotesters who support abortion rights — were already arriving Friday morning for an event that is expected to feature an address by President Trump, the first time a sitting president has attended.

People wearing “March for Life” sweatshirts crossed the Capitol grounds on the way to the march, along with others sporting red “TRUMP2020” baseball caps. Nearby, a separate group of counter protesters wearing sweatshirts that said “Literally, no one asked you” chanted “We love abortion, abortion is cool!”

The annual event protesting abortion started after the 1973 Rove v. Wade decision that legalized abortion in the United States.

Other Republican presidents have addressed the gathering by video, but none has attended. Mr. Trump tweeted on Tuesday: “See you on “See you on Friday … Big Crowd!” Friday…Big Crowd!”

The cameras in the Senate are government controlled by the Senate staff, and photographs are not allowed — limiting what viewers can see as lawmakers consider the case against President Trump. To get a more complete picture of the proceedings, here are two alternatives.

The Senate chamber may be familiar to viewers of C-SPAN, but the room has undergone some significant changes to accommodate the proceedings.

President Trump complained Friday that his lawyers would begin his defense on Saturday, a day the president said in the world of television was “called Death Valley,” as he unleashed dozens of tweets and retweets attacking the Senate trial.

The president began his social media assault just after 6 a.m. by retweeting Greg Jarrett, a conservative Fox News analyst, who was attacking the Democrats’ case. In one post, Mr. Jarrett accused Representative Adam B. Schiff of California, the lead House manager, of lying about the evidence.

Over the next several hours, he retweeted articles by breitbart.com; Lou Dobbs, the Fox Business Network host; Ben Ferguson, a conservative commentator; Dan Bongino, the host of a conservative radio talk show; and several Republican lawmakers, including Representative Kevin McCarthy of California, the minority leader in the House.

Later in the morning, Mr. Trump started tweeting his own attacks on the impeachment trial. In addition to complaining about the expected weekend start for his lawyers, Mr. Trump said he had “to endure hour after hour of lies, fraud and deception by Shifty Schiff, Cryin’ Chuck Schumer and their crew.”

The House managers prosecuting the case against President Trump will wrap up their arguments on Friday with a focus on the second article of impeachment: the accusation that the president obstructed Congress by blocking witnesses and documents in an attempt to cover up his misconduct.

It will be their last opportunity to appeal to a handful of moderate Republican senators on the question of seeking additional witnesses and documents before the president’s lawyers take center stage. Debate on that vital question is expected to happen early next week, after the conclusion of the arguments and a period of questions about the case from senators.

In the meantime, the Senate trial has tested the patience of senators, who have sat restlessly in their seats for more than 16 hours over two long days. Despite being admonished that they must remain silent and at attention “upon pain of imprisonment,” some have doodled, traded notes, whispered with their neighbors, or even nodded off.

Mr. Trump’s legal defense team is scheduled to begin their presentation on Saturday, angering the president, who complained on Twitter on Friday morning that “my lawyers will be forced to start on Saturday, which is called Death Valley in T.V.”

There have been discussions in the Capitol that senators could start the Saturday session earlier than the usual 1 p.m., which could give them the chance to leave earlier, especially if the White House lawyers decide to reserve more of their presentation for Monday.