One hundred senators are convening to solemnly consider whether impeached President Donald Trump should be removed from office for having committed high crimes and misdemeanors.

At least, that’s the theory.

But here in reality, the big question about Trump’s Senate trial is whether the House’s allegations will get any serious consideration from the chamber’s Republican majority — a majority that, at the trial’s conclusion, is all but certain to acquit the president and keep him in office.

With a prosecution and defense team, opening and closing arguments, the possibility of witness testimony, Chief Justice John Roberts presiding, and a mostly silent “jury” of 100 senators, the event has the trappings of a criminal proceeding.

But this is no ordinary trial. Senators are tasked with deciding whether to remove the US president from office — something that has never been done before.

And the Senate is no ordinary jury. They get to make important decisions about how the trial should be organized and unfold. Rather than being selected for their lack of previous knowledge of the case, they are 100 political animals with preexisting political beliefs, ambitions, and loyalties.

Some key Senate Republicans haven’t even tried to pretend the outcome is in question. “There is only one outcome that is suited to the paucity of evidence, the failed inquiry, the slapdash case,” Senate Majority Leader Mitch McConnell said last month. “I have clearly made up my mind,” said Sen. Lindsey Graham (R-SC).

On one level, then, the trial is about whether Trump is guilty of the offenses the House has charged him with — abuse of power and obstruction of Congress. Yet what’s really being determined is whether Donald Trump should continue to serve as president. So far, more than enough Senate Republicans clearly think the answer is yes.

1) What is a Senate impeachment trial?

The US Constitution gives Congress the power to hold federal officers accountable for high crimes and misdemeanors through the impeachment process. But that process unfolds in two steps. First, the House of Representatives decides whether to impeach — essentially, whether to charge with wrongdoing. (The House already did that for Trump.) Second, it’s up to the Senate to decide whether the official is guilty of that wrongdoing, for which the punishment entails removal from office.

The Senate does this by holding a trial. There have been 19 such trials in US history — 15 for federal judges, one for a senator (after which the Senate concluded that senators can’t be impeached), one for a Cabinet officer, and, most momentously, two for presidents (Andrew Johnson in 1868 and Bill Clinton in 1999).

Trump’s Senate trial will focus on the two articles of impeachment approved by the House. Article I, “Abuse of Power,” accuses Trump of pressuring the Ukrainian government into announcing an investigation into the Bidens by withholding both a White House meeting and military aid. Article II, “Obstruction of Congress,” accuses Trump of trying to impede the impeachment inquiry by urging government agencies not to comply with subpoenas and witnesses not to cooperate.

These trials typically end in a final vote on each article of impeachment. A vote to convict on even one article means the official is ousted. But, crucially, conviction requires a two-thirds supermajority in the Senate; a simple majority won’t do the trick. That means it would take 67 senators to remove Trump, and that’s a very high bar to clear. But if it is somehow cleared, Trump would be ousted, and Vice President Mike Pence would become president of the United States.

2) Are there prosecutors, defense, and a judge, like an ordinary trial?

A presidential impeachment trial is a rare proceeding involving all three branches of the federal government (and both houses of Congress), with a structure broadly resembling a criminal trial. The players are:

The House impeachment managers (the prosecutors): They are Reps. Adam Schiff (D-CA), Jerry Nadler (D-NY), Hakeem Jeffries (D-NY), Val Demings (D-FL), Jason Crow (D-CO), Sylvia Garcia (D-TX), and Zoe Lofgren (D-CA).

Trump’s team (the defense): The president has to designate representatives to defend him, as well. White House counsel Pat Cipollone will lead the team, and it will also include Trump’s personal lawyer Jay Sekulow, as well as Michael Purpura and Patrick Philibin from the White House Counsel’s Office as well as Ken Starr (the independent counsel who helped spur Bill Clinton’s impeachment), Robert Ray (who succeeded Starr in that post), and celebrity lawyer Alan Dershowitz.

Chief Justice John Roberts (the judge, sort of): The chief justice “shall preside” over a presidential impeachment trial, per the Constitution. But, as we’ll discuss, it’s not clear how substantive that role is.

The Senate (the jury, sort of, but also the court): The trial is conducted for the benefit of the 100 senators, who will eventually decide whether to remove the president from office or keep him there. And like jurors in a criminal trial, senators mostly aren’t permitted to speak at the trial; they’re supposed to observe silently.

However, unlike ordinary jurors, the Senate will make decisions on how to run the trial, with contested decisions being brought to a vote in the chamber. Since there are 53 Republican senators and 47 Democrats, the GOP will have the upper hand in making those decisions — so long as they can keep their moderate members in line.

3) How have previous impeachment trials played out?

Though the Senate has a set of impeachment rules, those rules do little to settle key questions about how Trump’s trial will be structured and proceed. How long will it take? What evidence will be admitted? Will witnesses be called? All these questions are really up to the Senate to decide.

We can expect proceedings to be bookended by opening and closing arguments from each side, as in a criminal trial. But what comes in between for Trump is yet to be determined, and it varied greatly for Andrew Johnson’s and Bill Clinton’s trials.

Johnson’s 1868 trial took nearly two months from start to finish. The impeachment articles there focused mainly on whether Johnson violated a new law, the Tenure of Office Act, by firing his secretary of war without the Senate’s approval. Several administration officials were called to testify by each side about Johnson’s actions. The outcome appeared genuinely in the balance as the Senate prepared to vote on a verdict, with all manner of behind-the-scenes shenanigans unfolding.

Clinton’s 1999 trial lasted about one month. He was accused of perjury and obstruction of justice in his effort to cover up his affair with Monica Lewinsky — and, due to his high approval rating and continued support among most Democrats, he was widely expected to be acquitted. Things kicked off on the Senate floor with televised opening arguments and a period during which each side answered written questions submitted by senators. Then the GOP-controlled Senate voted to approve the proposal of House impeachment managers to call three witnesses — Lewinsky, plus Clinton allies Vernon Jordan and Sidney Blumenthal — for videotaped closed-door depositions.

But that testimony (which was soon released publicly) didn’t produce any revelations, and it was clear the GOP lacked the votes to convict. So the Senate decided to pull the plug, voting to deny a request from House Republicans to call Lewinsky for another round of questioning and to schedule an end date for the trial without any new witnesses. After that, action resumed in televised Senate floor proceedings with a presentation of evidence from each side and closing arguments. What followed were several days of speechmaking by senators behind closed doors (with many senators subsequently releasing their remarks), and finally, public votes on a verdict for each article.

4) How long will the trial take?

The trial technically began with the ceremonial presentation of the articles of impeachment — but the real action kicked off on Tuesday, January 21.

The Senate began by voting on McConnell’s plan for structuring the trial, and on a proposed Democratic alternative. After that, opening arguments began, with the House impeachment managers going first, followed by Trump’s defense team.

McConnell reportedly said in private that he wants a short trial, lasting around two weeks, with no witnesses called to testify. Minority Leader Chuck Schumer, though, has publicly called for testimony from four witnesses with knowledge of the Ukraine scandal: acting White House Chief of Staff Mick Mulvaney, former National Security Adviser John Bolton, White House aide Rob Blair, and Office of Management and Budget official Michael Duffey.

McConnel locked down the Republican votes necessary to begin the trial without resolving the witness question (as the Senate did in 1999). Bolton’s recent announcement that he would comply with any Senate subpoena for his testimony made it harder for Republicans to justify skipping witnesses (though President Trump has also suggested he may try to block Bolton’s testimony through executive privilege). Whether McConnell will get his way remains to be seen, although it looks likely after key swing votes announced during the trial they would not call for more witnesses.

5) But McConnell is just going to call all the shots, right?

So long as McConnell can keep his party together, yes, he will essentially call the shots for Trump’s trial. And he has done that so far — though on certain issues, he might find it more difficult.

The math of impeachment votes is somewhat unusual for the Senate. The vote on a final verdict, as mentioned, needs 67 yeses to pass. But measures before that can be approved by a simple majority. However, Vice President Pence, who can usually break ties in the Senate, is not expected to vote on any impeachment matters. (He has a bit of a conflict of interest, since he’d become president if Trump is removed.)

For McConnell to pass a partisan measure, he’ll need to keep 51 of the 53 Senate Republicans together. That would mean he needs to keep at least one of the expected trio of swing votes — Sens. Collins, Lisa Murkowski (R-AK), and Mitt Romney (R-UT) — on his side.

Meanwhile, for Democrats to get a measure of their own passed, they’d likely need to win over four Republicans (47 Democrats plus four Republicans equals a 51-vote majority). In addition to the aforementioned trio, they’d need someone else. CNN has proposed several possibilities, but the incentives for partisanship are strong in the Senate.

And, to complicate things further, there’s the question of the chief justice.

6) What is Chief Justice John Roberts’s role?

The Senate impeachment rules give the presiding officer — in this case, Chief Justice Roberts — power to rule on “all questions of evidence,” specific questions from senators to be put to witnesses or counsel, and motions. That could give him enormous influence on how the trial plays out.

But there’s a big catch: The Senate can reject any of his rulings with a majority vote. Still, the specifics of Roberts’s ruling could influence how senators make up their minds (theoretically, key swing vote Republicans could be hesitant to overrule Roberts).

Roberts has precedent for a hands-on role. For the first presidential impeachment trial, that of Andrew Johnson in 1868, Chief Justice Salmon Chase demanded to play an active role, as Brenda Wineapple chronicles in her book The Impeachers. In addition to ruling on evidence and the scope of testimony, Chase even decided he would have the power to cast a tie-breaking vote in the Senate (something usually done by the vice president).

Chase cast two tie-breaking votes, but this was controversial at the time — and while the Senate let the votes stand, they’ve never quite officially said the chief justice can do this. (Asked whether Roberts would break ties, a spokesperson for McConnell seemed to signal otherwise to Vox’s Li Zhou, “Anything that is put to a vote during an impeachment trial, ties lose.”)

Yet Roberts also has a sharply different model he can follow. When the next presidential impeachment trial came around 131 years later, Chief Justice William Rehnquist’s main objective was to stay out the way. Rehnquist wanted to let the Senate take the lead and run Bill Clinton’s trial as they saw fit, so he declined to rule on contested issues. He’d just refer any controversial questions to the full Senate for a vote.

Indeed, Rehnquist’s sole “ruling” of note was that House impeachment managers should stop addressing senators as “jurors” — because, he said, the Senate is “not simply a jury” but “also a court.” After things wrapped up, he summarized his role by saying, “I did nothing in particular and I did it very well.”

So it’s up to Roberts whether he’ll interpret his role like Chase, Rehnquist, or somewhere in between. Importantly, though, the nature of the Supreme Court has changed since Chase’s time. Chase was a political animal — indeed, he was angling for the Democratic presidential nomination the same year of Johnson’s trial.

In contrast, Roberts wants himself and the Supreme Court itself to have the appearance of being above grubby partisan politics. That would seem to suggest he’d take on more of a restrained role, like that of Rehnquist. Then again, he could also think of himself as an impartial arbiter helping to broker compromises that make our political system more functional, and use that to justify intervening. Again, it’s up to him.

7) Will Trump show up and testify?

The president has claimed he would “strongly consider” testifying in the impeachment inquiry at some point. But most in Washington very much do not expect that to happen. Remember, Trump also frequently claimed he’d be happy to do an interview with former special counsel Robert Mueller during the Russia probe, but he ended up submitting just one written set of heavily lawyered answers.

Trump’s legal team is certainly hoping he won’t testify. And neither Johnson nor Clinton showed up to their impeachment trials, preferring to let their defense teams do the talking.

But Trump is known for wanting to take a prominent role in his own messaging. Plus, his acquittal certainly seems safe at this point, given the supermajority required for conviction and the existing GOP majority. What does he have to lose? Why not show up, give the Democrats a piece of his mind, and get some credit for his inevitable victory?

He probably won’t, though. Probably.

8) How will senators decide a verdict?

Unless there’s a successful motion to dismiss the charges against Trump, what lies at the end of all this are Senate votes on whether he’s guilty or not guilty of each charge (abuse of power and obstruction of Congress).

As presented, the question for senators may seem simple: Did Trump do what he’s accused of doing? But the true question lurking beneath this is: Should Trump be removed from the White House?

And that’s a higher bar. Take, for instance, the Clinton impeachment. Some senators believed he was, in fact, guilty of perjury and obstruction of justice but still concluded he should not be removed from office.

Politics naturally played a role. Clinton was overwhelmingly popular, large majorities of the public opposed his removal, and Republicans came to believe impeachment was a political loser after their weak performance in the 1998 midterms. But senators also invoked the greater good of the country, arguing it would be a mistake to remove a president the public clearly supported. (No Democrats voted to convict Clinton, and several Republicans joined them in voting to acquit.)

As for Johnson, one major factor in his acquittal was who would have replaced him: Ben Wade, the Senate’s president pro tempore (since Johnson had no vice president after succeeding the assassinated Abraham Lincoln). Wade was viewed even by many mainstream members of his party as an extremist (due to, among other matters, his support for voting rights for freedmen and even women). Convicting Johnson meant making Wade president, and some Republicans feared that would sink the chances of their nominee for president in the election later that year: Ulysses S. Grant.

There may have been baser motives at play, too. After Johnson was acquitted by a single vote (due to a late switch by Sen. Edmund Ross), the House immediately launched an investigation into whether key senators had been bribed. In Wineapple’s telling, the investigation found “a great deal of smoke” but couldn’t outright prove bribery.

For Trump, though, the likely determining factor will be simple partisanship. The president is unpopular overall, but the vast majority of Republican voters continue to strongly support him. The impeachment inquiry and its revelations about quid pro quos haven’t changed that. The safe bet for senators who seek a political future in the GOP continues to be to stand by Trump.

9) Is there even a chance that Trump gets removed? Or charged criminally?

As far as criminal charges go, the articles of impeachment the House approved for Trump are not ordinary federal crimes. Rather, they’re constitutional “high crimes and misdemeanors.” Whether Trump committed any criminal wrongdoing in connection to the Ukraine scandal would be up to the Justice Department to determine. And, they’ve said, they won’t charge a sitting president with a crime. (Should Trump lose reelection in 2020, though, it’s possible that a Justice Department of his successor, under new leadership, would scrutinize his conduct.)

As for removal from office in the trial — well, never say never. But Trump technically needs only 34 Senate supporters to vote for his acquittal. There are 53 Senate Republicans, which means that at least 20 would have to defect and vote to convict him.

For that to happen, there would have to be some truly shocking developments. Keep in mind that almost all of these Senate Republicans have spent the past few years supporting Trump and standing by him — and that zero Republicans in the House voted to impeach him.

Despite the damning facts in the Ukraine scandal, Republicans have offered a variety of justifications for keeping Trump around. Some say they’re just respecting the will of the voters (in key Electoral College states); some say this is all a partisan plot from Democrats; some say Trump did nothing wrong; some say the House just didn’t get enough evidence.

It’s not inconceivable that new significant evidence will emerge. But even then, it would take a true earthquake to result in 20 Republican defections. What would likely be required is Trump’s popularity finally plummeting among Republican voters. So far, though, he has avoided that fate — and unless that changes, he’s overwhelmingly likely to finish out his term.