It’s been three weeks since the House of Representatives voted to impeach President Trump, and yet the outlines of the process that will decide whether he remains in office are still stuck in political limbo. Republican leaders are calling for a swift, minimalist trial while Democrats demand new witnesses and evidence. And on Monday, it seemed as if Democrats might actually be able to nab a high-profile witness. Former national security advisor John Bolton announced that he would finally testify before Congress if subpoenaed by the Senate, introducing the possibility that new and potentially damaging evidence against Trump could still appear, perhaps shifting the narrative and public opinion on impeachment.

But Republican Senate Majority Leader Mitch McConnell looks unlikely to cooperate, or at least, unlikely to make a decision on whether to call new witnesses immediately. On Tuesday, he said he had the votes to open the trial now in spite of Democrats objections, arguing that he’s just following the precedents set by the Clinton impeachment trial.

This negotiation and ambiguity might seem bizarre or unfair to observers, but it’s actually part of the Constitution’s design. The Constitution gives virtually no guidance about what an impeachment trial is supposed to look like, and the courts have also said they can’t weigh in. There are Senate procedures on how impeachment trials should run, but they’re fairly vague, providing more guidance about how decisions are made than the substance of the trial itself. So just as Republicans had little control over the impeachment inquiry in the House, Democrats are now faced with a process where a majority of Republican senators can set the rules of the road.

When most people think of a trial, they probably imagine a courtroom with a judge in black robes, a box with rows of jurors, and tables for the defense and prosecution. The trouble with understanding how an impeachment trial works is that all of these components are present in some form — for example, a few House lawmakers appointed to be impeachment managers will serve as prosecutors, and Trump’s legal team will offer a defense. Those parallels can make an impeachment trial seem superficially similar to its criminal or civil counterparts, but the process is different in fundamental ways. For one thing, as I wrote a few months ago, Trump doesn’t have a right to protections like due process because he hasn’t been charged with a crime.

Meanwhile, while ordinary legal trials follow a set of detailed procedures, in a 1993 case involving the impeachment of a federal judge, the Supreme Court ruled that the Senate has free rein to make its own rules without courts getting involved — including how quickly the trial proceeds, which stages will be public or private, and whether or how evidence can be admitted. (Chief Justice John Roberts will preside over the trial and can rule on questions of evidence and break ties, but his actions can be overridden by a majority of senators.)

Another key aspect of how an impeachment trial is different from other legal proceedings is that senators don’t just have to determine whether the facts add up to a specifically defined crime, they also have to determine which facts are relevant to begin with. That’s because there’s no clear definition for what an impeachable offense is under the Constitution, or at least not in the way a criminal statute would define wrongdoing. And when the senators are deciding whether to convict or acquit the president, it’s up to them to decide which standards of evidence they’ll apply to determine whether Trump is guilty.

Senators also aren’t really called upon to be impartial in the same way jurors ordinarily are. The Constitution does specify that the senators “shall be on oath or affirmation,” but it doesn’t say more about what that means. In the oath, senators swear to “impartial justice according to the Constitution and laws.” But what that means in practice is pretty much up to each individual, as there’s no clear mechanism for disqualifying a senator from participating in the trial because of a lack of neutrality, according to Sarah Binder, a political science professor at George Washington University.

And in the end, even though there are rules to guide the process, most of the key decisions will be made by a simple majority vote. “It all comes down to what 51 elected senators want to do,” said Frank Bowman, a professor at the University of Missouri School of Law and the author of “High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump.” That means for better or worse, there are few objective standards for either side to lean on as they move forward — and plenty of room for dispute about what a fair trial really looks like.

In the past, questions of fairness and impartiality haven’t been as heated, largely because the rule-setting process simply wasn’t as partisan. The initial procedures for Clinton’s impeachment trial, for instance, were subject to intense negotiation but then adopted by a unanimous vote. But even though McConnell has said he’s just following the example set in Clinton’s impeachment trial by punting the question of witnesses until the next stage, following past precedents won’t end the debate over new evidence, since additional witnesses appeared in both of the country’s previous presidential impeachment trials. It will be hard for McConnell to argue that an impeachment trial without witnesses is the norm.

Admittedly, the scope in testimony in the previous two cases was very different — more than 40 witnesses testified publicly before raucous and overflowing crowds during the 19th-century trial of President Johnson, while only three witnesses were privately deposed during President Clinton’s trial in 1999. But Philip Bobbitt, a professor at Columbia University Law School and the co-author of “Impeachment: A Handbook,” said that McConnell’s attitude toward the trial is unusual. “What we have here is a Senate majority essentially acting like a criminal defense attorney trying to bar testimony.” But, he added, Democrats also have little recourse if McConnell manages to keep his majority in line. “There’s no higher power [Democrats] can appeal to,” he said. “Impeachment is a system that depends on the judgment of the officials who enforce it and the people who elect them.”

There is one bright spot for Democrats: Their position is currently more popular than McConnell’s. According to our recent poll with Ipsos, a majority of Americans think it would be better if the upcoming trial included new witnesses who could potentially shed light on Trump’s conduct, while only 39 percent said it would be better to keep the focus solely on the evidence introduced in the House hearings, without calling new witnesses.

Binder said Democrats can try to force votes on whether to call witnesses as the process moves forward, too, which could keep the spotlight on the issue. But ultimately, once the articles of impeachment are transmitted to the Senate, disputes over fairness in the impeachment trial will mostly be decided by majority rule. That might not be the kind of trial most people are used to — but it’s the one the Constitution mandates. And it means even after the trial starts, the debate over how rules and witnesses will likely be far from over.