As the answers to: How many Senators must vote to override specific judgments of the Chief Justice in an impeachment trial of the President? state, the rules of the Senate Impeachment trial are set by the Senate itself in a resolution that has to pass by a simple majority. So if a majority wants to make the trial take a long time, they can pass rules that would support that, or they could pass rules that would let them wrap it up as quickly as possible.
What are the rules?
Essentially, it’s up to senators to decide the format of the trial, but they will most likely follow a set of rules governing impeachment trials that were revised in the 1980s.
Mr. Schumer laid out a detailed proposal this week that called for additional witnesses who have not previously testified (including Mick Mulvaney, Mr. Trump’s acting chief of staff, and John R. Bolton, the president’s former national security adviser) as well as subpoenas for additional documents that the White House has withheld.
Mr. McConnell rejected Democrats’ demands to call White House officials as witnesses and said he had no obligation to be evenhanded or impartial during the proceedings. He is working with the White House to establish rules for the trial, trying to balance Mr. Trump’s desire for vindication with lawmakers’ concerns.
“When the Senate decided what the rules were going to be for our trial, they really made them up as they went along,” Gregory B. Craig, who helped defend Mr. Clinton in his impeachment proceeding and later served as White House counsel to President Barack Obama, told The New York Times in 2017.
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The rules can also be changed during the trial by majority vote:
Under impeachment trial rules, any senator can force a vote on a motion, with a simple majority needed to be successful. Unlike normal Senate votes, Vice President Pence cannot break a tie. That means Schumer needs four GOP senators to successfully call witnesses or request documents. He needs only three Republicans if he wants to block a GOP motion because, according to senators, a 50-50 vote would result in a motion failing.
Schumer aims to drive wedge between Republicans on impeachment
Here’s another source that might help explain the rules of a Senate impeachment trial:
Regardless of how it begins or ends, the trial itself is governed by standing Senate rules, last modified in 1986. They are largely based on precedents set in the Andrew Johnson trial. ... Witnesses are called and cross-examined according to protocols and timelines adopted by the Senate just before the trial begins on a majority vote. During the Clinton impeachment proceedings, they were (somewhat miraculously) adopted by unanimous consent, in part because the outcome of the trial was in zero doubt.
As I noted recently, the scope of the trial, and rules for calling, examining and cross-examining witnesses, are really up in the air, and may depend on how disciplined Republicans (who hold 53 Senate seats) are in doing what the White House and Mitch McConnell tell them to do:
Chief Justice John Roberts, who according to the Constitution will be sworn in as the chief presiding officer (but not the “judge” — the Senate itself is the judge and the jury), can rule on matters of procedure and evidence, but can also be overruled by 51 senators at any time. What will not be in play are the normal rules of evidence in criminal and civil trials; impeachment trials are their own beast.
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