Updated at 12:57 p.m.
President Trump's impeachment trial moves Wednesday to its next phase: written questions from senators.
For up to 16 hours spread over two days, senators from both parties will submit questions to House managers prosecuting the case as well as Trump's defense lawyers; U.S. Chief Justice John Roberts, who is presiding over the trial, will read the questions aloud on the floor of the chamber.
Watch the proceedings live when the begin at 1 p.m. ET.
This step in the proceedings comes a day after the president's legal team rested in its defense of Trump. Democratic House managers made their case last week on why the president should be removed from office. Trump was impeached last month on charges of abuse of power and obstruction of Congress.
During the impeachment trial of President Bill Clinton in 1999, senators asked more than 150 questions over two days.
Republicans and Democrats reached a deal in which the parties will take turns asking questions for up to eight hours Wednesday. The same rules and duration for questions has been allotted for Thursday.
Among those who be watching the session Wednesday is Joseph Bondy, the attorney for Lev Parnas, the former associate of Rudy Giuliani. NPR's Sue Davis says Bondy will be in the Senate gallery as a guest of Minority Leader Chuck Schumer, D-N.Y. Parnas, who has been indicted in New York for alleged campaign finance violations, won't be in the gallery because he's wearing a GPS monitoring device as part of his pretrial release. Bondy declined to say where Parnas will watch the proceedings from.
The two days of Senate questions set the stage for a Friday vote on whether to allow witnesses.
According to Democrats, multiple potential witnesses have firsthand accounts of what they say is a quid pro quo scheme with Ukraine that forms the basis of the two impeachment charges Trump is facing.
Democrats say witnesses are essential for a fair trial, while Republicans maintain that witnesses can inject uncertainty into the proceedings and prolong the trial.
Sen. Pat Toomey, R-Pa., told NPR's Claudia Grisales that he remains "very, very skeptical that there is any witness that is going to shed any light that is going to cause me to change my view on what the final outcome of what this trial ought to be." Republicans overwhelmingly support Trump's acquittal.
Among the sought-after witnesses is former national security adviser John Bolton, who captured Washington's attention this week following the release of revelations in his forthcoming book that Trump allegedly told him that military assistance was only to be released after Ukraine opened investigations into Trump's Democratic rivals. Those purported book details, first reported by The New York Times, placed pressure on senators to vote to allow witnesses into the trial.
Trump's defense team, in response, argued that a manuscript of Bolton's book would be "inadmissible" in the trial. Defense lawyer Jay Sekulow said impeachment "is not a game of leaks and unsourced manuscripts." Anything Bolton would have to offer in person about his conversations with the president, argued Trump attorney Alan Dershowitz, would not rise to the level of an impeachable offense.
Whether Republicans can defeat the Democrats' push for witnesses, including Bolton, is unclear.
Senate Majority Leader Mitch McConnell told Republicans in a closed-door meeting late Tuesday that the party does not yet have the votes to block new witnesses, but that does not mean Democrats will be able to subpoena Bolton or any other witness. That decision will likely come down to moderate Republicans including Mitt Romney, R-Utah; Lamar Alexander, R-Tenn.; Lisa Murkowski, R-Alaska; and Susan Collins, R-Maine; who have indicated they may be open to hearing from witnesses, but their final decision is still unknown.
Trump's defense team argued earlier this week that the case against the president presented by Democrats does not constitute any impeachable offenses.
Lawyers for Trump have accused Democrats of trying to invalidate the result of the 2016 election. They say Trump has not committed any crime and therefore cannot be removed from office, even though many legal scholars say a law need not be broken in order to secure an impeachment conviction that would remove a president from office.
Although two previous presidents, Clinton and Andrew Johnson, have also been impeached, no U.S. president has ever been removed from office.
Democrats impeached Trump last month for abuse of power and obstruction of Congress for what the prosecution team says was a campaign to pressure Ukraine into opening investigations into Trump's political rivals and held-up $391 million in congressionally approved security assistance in order to apply leverage on Ukrainian authorities. House Democrats say the White House's refusal to cooperate with the impeachment investigation is proof of a cover-up and amounts to obstruction.
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