By Jeremy Herb, Manu Raju, Phil Mattingly and Lauren Fox | CNN

Senators finally got their chance to participate in President Donald Trump’s impeachment trial by asking questions Wednesday — a phase both sides are hoping can sway their undecided colleagues on the issue of witnesses.

In a sign of the importance of their role, Senate Republicans gave the opening question of the day to Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah, three Republicans who have hinted they could vote to hear from witnesses. Collins’ question, submitted through Chief Justice John Roberts, asked the President’s legal team how senators should consider if the President had multiple motives when he held up US security aid to Ukraine.

“If there is any possibility, if there is something that shows a possible public interest and the President could have that possible public interest motive — that destroys their case,” said Patrick Philbin, deputy counsel to the President. “So once you’re into mixed-motive land, it’s clear that their case fails.”

Senate Democrats teed up lead impeachment manager Rep. Adam Schiff of California to respond on the next question — complete with videos of the defense counsel presentation to argue that the Senate should hear from former national security adviser John Bolton.

“If you have any question about the President’s motivation, it makes it all the more essential to call the man who spoke directly with the President, that the President confided in and said he was holding up this aid because he wanted Ukraine to conduct these political investigations that would help in the next election,” Schiff said. “Don’t wait for the book.”

Later in the afternoon, Collins and Murkowski asked a question about whether the President had raised with his aides the issue of corruption in regards to Joe Biden and his son Hunter Biden before the former vice president entered the presidential race. Philbin said he didn’t know of specific conversations with Trump, and instead discussed the investigative efforts of the President’s attorney Rudy Giuliani in Ukraine.

The back-and-forth over Trump’s motives and Bolton kicked off two days of senator questions to the House managers and the President’s legal team, in the final phase of the trial before the Senate will debate and vote on whether to seek witnesses and documents. Senators still can’t speak during the trial, so they are written on a notecard and Roberts reads them aloud.

Key senators listening in

The questions provided some insight into the thinking of senators after hearing three days of uninterrupted presentations from both sides, although most of the questions were friendly: Democrats questioning the House managers and Republicans asking for a response from the President’s team.

When Philbin began answering a question from Sen. Rob Portman, an Ohio Republican who hasn’t stated his position on witnesses, about the implications for the Senate if they move forward on witnesses, Sen. Lamar Alexander of Tennessee began taking extensive notes. Philbin contended that such a step would set a dangerous precedent and paralyze the body.

Alexander, viewed by both sides as potentially the pivotal vote on witnesses, continued taking notes as Schiff made his counterpoint.

One of the more notable answers came from Alan Dershowitz, the Harvard law professor emeritus on the President’s legal team, who argued that a quid pro quo to boost a president’s reelection chances cannot be impeachable because the politician is acting in the national interest.

Senate leaders in both parties have worked with senators to craft questions that will be asked, as Republicans push for an end to the trial while Democrats make their case for calling witnesses and subpoenaing documents.

The vote on witnesses, which is expected Friday, comes down to the handful of Republicans who remain undecided on whether there should be witnesses called in the trial. Republican senators met behind closed doors on Tuesday, where Senate Majority Leader Mitch McConnell told his conference they didn’t have the votes — yet — to defeat witnesses, but Republicans left the meeting confident they can get there.

“There have been a lot of discussions, but I have no idea how the votes are going to fall,” said Collins, a Republican who is leaning toward voting for witnesses, before the trial began on Wednesday. “I am pleased that I, along with Mitt Romney, Lisa Murkowski and others, worked very hard to get it into the resolution a guaranteed vote on whether or not to call witnesses at this point in the trial.”

McConnell met on Wednesday morning with Murkowski, an Alaska Republican also on the fence about witnesses.

“I had a meeting with Leader McConnell, but I’m not going to talk to you about it,” Murkowski said after leaving McConnell’s office. “I am not going to be discussing the witness situation right now. … I’ve got some more questions that I want to get into the mix. So I’ve been talking with the folks in the cloakroom about what the universe is, see how we can supplement that, so that’s my purposes.”

Wild cards in both parties

Perhaps the biggest wildcard right now is Alexander, the retiring Tennessee Republican and Senate institutionalist who is close to members on both sides. He is also a McConnell confidant.

Sources who attended Tuesday’s Republican meeting say Alexander did not speak about his views on witnesses — and is not tipping his hand one way or the other. He is reading through the materials from the inquiry and past testimony and continues to say he will make a judgment after the questioning period is done.

On the Democratic side, there are still a handful of red-state senators who have not said how they will vote on whether to convict or impeach Trump on the articles of impeachment.

Sen. Joe Manchin, a West Virginia Democrat, has said he wants witnesses, like former national security adviser John Bolton. On Wednesday, Manchin said he was also potentially interested in a key witness sought by Republicans: former Vice President Joe Biden’s son, Hunter Biden.

“I want witnesses. I definitely want witnesses,” Manchin said. “The only thing I’ve said is that there should be an adult in the room and that’s Chief Justice Roberts. We should vote again on Chief Justice Roberts being able to determine who is pertinent … if Hunter Biden is one of the people who is pertinent to the evidence or to the trial, then absolutely.”

Manchin has not tipped his hand on his final vote, but said both sides did a “good job” with their presentations.

Trump suggested on Twitter that Manchin would ultimately vote against him.

“There is much talk that certain Democrats are going to be voting with Republicans on the Impeachment Hoax, so that the Senate can get back to the business of taking care of the American people,” Trump tweeted. “Sorry, but Cryin’ Chuck Schumer will never let that happen!”

Questions to help make the case

Questions alternated between Republicans and Democrats, with a Senate aide walking the written queries up to the dais for Roberts to read them aloud.

A majority of the questions could be characterized as softballs: Senate Republicans gave the President’s team the chance to explain its case, and Senate Democrats did the same for the House impeachment managers. There were questions posed to both sides asking them to simply respond to what the opposition had just said.

The pace had changed from the often lengthy presentations that were delivered during opening arguments, as Roberts held each side to a five-minute response to the questions — cutting off those who went over their time.

Democrats spread out their questions to the House managers, with Schiff, the head manager taking the lead. Philbin answered a majority of the questions for the President’s team, with others chiming in occasionally.

Still, there were some notable responses, such as when Sen. Ted Cruz, a Texas Republican, asked the President’s team, “As a matter of law, does it matter if there was a quid pro quo?”

Dershowitz responded that a politician trying to win reelection is acting in the national interest, saying that Trump cannot be impeached for pressuring Ukraine for investigations into Joe Biden because the President’s motivations would ultimately be fueled by the public interest, if he believes his reelection is what’s best for the country.

“Every public official that I know believes that his election is in the public interest,” Dershowitz said. “And if a president did something that he believes will help him get elected, in the public interest, that cannot be the kind of quid pro quo that results in impeachment.”

Senate Minority Leader Chuck Schumer, a New York Democrat, asked Schiff to respond. “All quid pros are not the same,” Schiff said. “Some are legitimate and some are corrupt, and you don’t need to be a mind reader to figure out which is which.”

Occasionally, senators quizzed the lawyers on the other side of the aisle. Sen. John Kennedy, a Louisiana Republican, posed a question to both teams about why the House hadn’t challenged the President’s executive privilege claims. Sens. Lindsey Graham of South Carolina and Cruz asked the House managers a hypothetical about former President Barack Obama asking Russia for an investigation into his then-opponent, current Utah Sen. Mitt Romney.

Sen. Martin Heinrich, a New Mexico Democrat, asked the President’s team when the White House counsel had learned about the Bolton manuscript. Philbin said they had been notified of it, though he did not say when. And independent Sen. Angus King of Maine, who caucuses with the Democrats, asked both sides about former White House chief of staff John Kelly’s comments that he believes Bolton and he should testify. “Do you agree with General Kelly?” King asked.

Jay Sekulow, Trump’s personal lawyer, ticked off the denials from the President, the vice president and the Justice Department.

“To move that into a change in proceeding, so to speak, I think is not correct,” Sekulow said, adding that the President’s team expects witnesses if the House gets witnesses.

Schiff urged senators to hear from Bolton so they could judge for themselves.

“It’s really, at the end of the day, not whether I believe John Bolton, or whether General Kelly believes John Bolton,” Schiff said, “but whether you believe John Bolton — whether you’ll have an opportunity to hear directly from John Bolton, whether you’ll have an opportunity to evaluate his credibility for yourself.”