Sen. Susan Collins of Maine joined Republican colleagues Wednesday in questioning attorneys as the president’s impeachment trial moved to a new phase and closer to a highly anticipated vote on whether witness testimony should be allowed.

Collins’ inquiries included the opening question in Wednesday’s proceedings, in which she joined Republican Sens. Mitt Romney of Utah and Lisa Murkowski of Alaska in asking how senators should weigh the president’s motives in assessing the House’s charge of abuse of power.

Collins and Murkowski also asked Wednesday whether Trump was interested in Joe Biden’s connection to corruption in Ukraine before Biden entered the 2020 presidential race.

The three senators are seen as possible swing votes who could join Democrats in voting for witness testimony to be included in the trial later in the week. That vote will likely take place Friday.

Wednesday marked the start of a period for senators to ask questions of the president’s attorneys and House managers. The questioning could last up to 16 hours and is expected to go through Thursday, with senators submitting their questions in writing and Chief Justice John Roberts reading them aloud. Responses are limited to five minutes.

The first question, asked by Collins, Murkowski and Romney, was, “If President Trump had more than one motive for his alleged conduct, such as the pursuit of personal political advantage, rooting out corruption and the promotion of national interests, how should the Senate consider more than one motive in its assessment of Article I (abuse of power)?”

White House lawyer Patrick Philbin responded by refuting the idea that a mixed motive could serve as the basis for an impeachable offense, because it would be impossible to determine to what degree a decision was motivated by personal interest versus the interests of the country.

“That’s the standard they would have to meet – showing there’s no possible public interest and the president couldn’t have had a smidgen even of public interest motive,” Philbin said. “(The House managers) recognize that once you get into a mixed-motive situation – if there’s both some personal motive and also a legitimate public interest motive – it can’t possibly be an offense.”

Collins’ second question, asked with Murkowski, was about whether Trump spoke with the former Ukrainian president, other Ukrainian officials or his own aides or cabinet members about Joe Biden in connection to corruption in Ukraine before Biden announced he was running for president in April 2019.

Philbin said that information wasn’t thoroughly pursued in the House record, but there is other information “publicly available and in the record” about allegations Biden pressured the former Ukrainian president into dismissing a top government prosecutor and that Trump’s private attorney, Rudy Giuliani, had been investigating Ukraine before Biden’s announcement.

Sen. Angus King, an independent from Maine, provided a list of the 13 questions he had submitted to Democratic leaders for consideration as of Wednesday afternoon.

The question process is fluid, with senators being able to add questions as the questioning unfolds and party leaders filtering them out and deciding which ones get asked or combining duplicate questions asked by multiple senators.

Four of King’s questions had been accepted for inquiry and two of them had been asked at the trial as of 10 p.m. Wednesday.

The first question, directed to both the House managers and the president’s counsel, referenced a statement reportedly made by former Trump Chief of Staff John Kelly in which he suggested that former National Security Adviser John Bolton should testify, and asked whether the parties agreed witnesses should be heard.

In the second, King asked the House managers if President Trump’s mention of Rudy Giuliani in the July 25th phone call conclusively established the purpose of the call since Giuliani has said publicly that he was in Ukraine on a political errand.

Collins’ office would not provide a list of the questions she has submitted to be asked at the trial.

Collins, who is up for re-election this fall, is expected to face a tough and expensive race. Her seat has been targeted by Democrats nationally in part because of her vote to confirm Supreme Court Justice Brett Kavanaugh.

As the trial has unfolded, Collins again has found herself in a difficult position as one of a handful of moderate Republicans who could possibly side with Democrats in seeking to call witness testimony.

On Monday, she reacted to reports of an unpublished book manuscript by Bolton that ties Trump’s withholding of military aid to Ukraine to his request the country investigate Democrats, saying the book “strengthens the case for witnesses.”

She also has advocated for the trial to follow the same model as Clinton’s impeachment trial in 1999, in which she argued at the time for the inclusion of documents and witness testimony.