House Speaker Nancy PelosiNancy PelosiSen. Ron Johnson hoping for Democratic 'gridlock' on reconciliation package Virginia race looms as dark cloud over Biden's agenda Biden struggles to rein in Saudi Arabia amid human rights concerns MORE (D-Calif.) is poised to send the articles of impeachment against President TrumpDonald TrumpRobert Gates says 'extreme polarization' is the greatest threat to US democracy Cassidy says he won't vote for Trump if he runs in 2024 Schiff says holding Bannon in criminal contempt 'a way of getting people's attention' MORE to the Senate, setting in motion his long-awaited trial.
But don’t expect the Senate proceedings to mirror what happens in a court of law.
The Constitution gives the Senate “the sole Power to try all Impeachments” but is silent about the trial’s mechanics. In practice, Senate proceedings have come to differ dramatically from court trials on everything from the admissibility of evidence, the form of punishment and the possibility of appeal.
Here are five big differences to understand as the Senate prepares to begin Trump’s impeachment trial.
Senators are both jury and judge
In a court trial, the jury plays a largely passive role for much of the proceeding. Jury members typically spend the bulk of their time hearing evidence, before being asked to render a verdict.
A judge, on the other hand, actively shapes the trial by interpreting and applying the governing rules and procedures. Judges rule on matters such as the admissibility of evidence and other legal issues that can define the trial’s contours in key ways.
Most importantly, in a trial the jury and judge are two separate entities.
During a Senate impeachment trial, however, these two functions merge into one single body consisting of the upper chamber’s 100 senators, who act as both jury and judge.
The Constitution calls on the Supreme Court’s chief justice to preside over a presidential Senate impeachment trial. But the standing Senate rules make clear it is the lawmakers themselves who have ultimate authority over all critical aspects of the proceeding.
“The senators have power over how the trial unfolds,” said Ian Ostrander, a political science professor at Michigan State University. “They are not passive jurors.”
There is no standard of proof
The standard of proof refers to the obligation that a plaintiff or prosecutor has to meet to effectively prove their case at trial.
In a civil case, the burden to be carried is a “preponderance of evidence,” meaning a greater than 50 percent chance the assertion is true. In criminal trials, the measure is guilt “beyond a reasonable doubt.”
But in Senate impeachment trials no such burden, or standard, exists.
“There is no established or uniform burden of proof,” said Michael Gerhardt, a University of North Carolina law professor and author of “Impeachment: What Everyone Needs to Know.” “In the Senate, every senator decides for himself or herself what burden applies.”
There are no rules of evidence
Court trials usually involve rules of evidence, particularly when juries are involved.
The applicable rules vary from court to court, with some tribunals using the entire Federal Rules of Evidence, which cover everything from relevance to standards for determining the admissibility of expert opinion and exceptions to the general prohibition against hearsay.
“In any court there’s going to be some set of rules about what evidence can be received or not,” said Frank Bowman, a University of Missouri law professor and author of “High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump.”
“That’s not what you’ve got in impeachment,” Bowman continued. “There aren’t any rules of evidence.”
An important distinction is that an impeachment trial is more of a political rather than legal exercise. So, unlike in a criminal trial, there is no requirement that a president be charged with an actual crime.
This means that as senators consider the two articles against Trump — for abuse of power and obstruction of Congress — they face a vastly more open-ended and less structured assessment of wrongdoing than that of a trial jury.
That is already manifesting itself in a fight between Democrats and Republicans over whether to allow witnesses and when to make those decisions.
“Every senator gets to decide for himself or herself what are the facts and the law,” Bowman said.
The punishment is political
When criminal defendants lose their case, they face the loss of liberty or property, and in jurisdictions that allow the death penalty, loss of life. In civil cases, the punishment usually takes the form of a defendant paying money to the plaintiff.
But just as an impeachment proceeding is political in nature, so is its final sanction.
Trump is expected to be acquitted by the Republican-controlled Senate, where a two-thirds vote is required for a conviction. But in the highly unlikely event that he is convicted, Trump’s penalty would be removal from office, a remedy with far-reaching political implications.
“At stake is not only whether the current president stays in office, but the lessons to future presidents as to what they can, and cannot, get away with,” said Charles Tiefer, a University of Baltimore law professor.
The outcome can’t be appealed
In legal proceedings, it is common for a losing party to appeal a determination at trial up to a higher court. But the Constitution endows the Senate with the “sole” authority over impeachment trials, so there is no higher body to hear an appeal, some experts say.
“That means if there are perceived procedural irregularities, or a senator is perceived to violate his or her oath, there is no recourse in the judicial system,” said Robert Tsai, a constitutional expert and law professor at American University.
According to Tsai, the only available remedy against a senator who violated their oath to render impartial justice during the trial would be to seek to expel a senator, or simply seek to have them voted out of office.
Other impeachment scholars say that while judicial review of the Senate trial result is generally unavailable, there are some limited circumstances when a court can step in.
The Constitution requires that senators swear an oath before the proceeding, and long-standing Senate rules on impeachment trials describe that oath as a promise to “do impartial justice.”
“If a senator does not make the oath or makes a false oath, that should be reviewable,” said James Robenalt, an attorney with the firm Thompson Hine and an expert on Watergate.