This post has been updated.

Republicans are decrying the impeachment process in the Democratic-controlled House, but if President Trump is impeached, they’ll have almost all the power to decide whether he’s innocent or guilty and should be removed from office. One Republican in particular: Senate Majority Leader Mitch McConnell (R-Ky.).

As leader of the Senate, McConnell will have broad latitude to call witnesses who may be less damaging to Trump. He can speed up the trial or slow it down, and he can even circumvent some of the rules as dictated by the overseer of all this, Chief Justice John G. Roberts Jr.

“He can frame this trial however he wants,” said Sarah Burns, a constitutional law expert at Rochester Institute of Technology who has studied this, “because it’s not an entirely legal process and doesn’t adhere to the legal rules. So he has a lot more leeway to do what’s in his political interest.”

In some ways, McConnell and, through him, Senate Republicans have more power than House Democrats to shape Trump’s future.

Here’s how: McConnell would need a supermajority to change the trial rules that the Senate approved in the 1980s. But to just overrule Roberts’s interpretation of the rules, he would need a simple majority. He couldn’t easily blow up the trial process, but he can tweak around the edges, Burns said.

For example, Burns said he could severely restrict House lawmakers (called “managers”) from calling witnesses. Or he could give himself power to call witnesses and make sure Trump allies, like Secretary of State Mike Pompeo, are on the stand. He could have Hunter Biden come testify and muddle who’s really on trial. “Essentially what we have here is the capacity of individuals in the Senate to change rules to undermine the constitutional system,” Burns said.

McConnell’s disputed this in a statement, saying “He does not have any of the powers described.”

Some law experts have argued it’s Roberts who calls the shots, but Burns argues that’s the wrong way to look at things. She says Roberts is just there to interpret procedure. That’s not unimportant, she said, but it also gives McConnell some open road to steer the trial into the lane he wants it since Roberts isn’t like a judge in a traditional criminal trial with unquestioned authority. The Senate can overrule Roberts with a 51-vote majority.

An updated report from the nonpartisan legislative experts at the Congressional Research Service also suggests the Senate has a lot of flexibility in the way it conducts the impeachment trial. The extreme nature of impeachment “could allow a simple majority to determine the procedures for responding to articles of impeachment sent from the House,” its authors write.

Of course, that doesn’t mean McConnell can unilaterally change whatever he wants. He still needs at a minimum a majority vote to do it. And he may simply choose to abide by the rules as written.

There’s evidence that’s the case. Sen. John Cornyn (R-Tex.), who works closely with McConnell, told the New York Times’s Carl Hulse: “I think he will play it straight.”

I asked Burns what’s the one thing to keep in mind as we follow a likely impeachment trial. She said it’s that the Senate trial is not a criminal trial. It’s a process unfamiliar to Americans who don’t remember the trial that followed President Bill Clinton’s impeachment, and we can’t try to put it in a more familiar box. McConnell is not the defense and Roberts the judge. The senators are both the judge and the jury. And Trump is not on trial for a crime, but rather for Congress’s interpretation of what he did wrong as president.

Adding to the confusion, a Senate impeachment trial is a really rare thing. There have only been 16 Senate impeachment trials in American history, two of presidents. Not a single president has been convicted. We don’t have a good frame of reference for a presidential impeachment trial, so it’s hard to judge what will be normal and what will be out of the norms. That probably also plays to McConnell’s benefit.

McConnell doesn’t have free rein to do whatever he wants. As Bruce Ackerman at Yale points out in Slate, he still needs a majority vote to change any of the rules, and it’s not a given that a majority of Republicans will stay with him.

He can only afford two defections, and two Republican senators, Mitt Romney of Utah and Lisa Murkowski of Alaska, have openly criticized Trump’s actions on Ukraine. So if McConnell wants to bend the rules in a way that asks the Republican Party to overrule Roberts, we will have to take it on a case-by-case basis whether he’d have the votes.

I’m talking about much of this in the abstract because we don’t yet have a clear understanding of how a battle over Senate trial rules might manifest.

And we don’t fully know McConnell’s motives. My Washington Post colleagues have talked to GOP strategists who say that above all else — even above having a Republican president — McConnell wants to keep his Senate majority in elections next year. Could he decide between now and then that ditching Trump is a tough-but-necessary move for him? Then he could frame the trial in a way that prosecutes Trump to the fullest extent.

But for now, all indications from McConnell are that he wants to protect Trump. And if/when the House impeaches the president, he’ll have significant authority to do it.

This post has been updated with a response from Sen. McConnell’s office; and with a correction: We previously referred to a new Congressional Research Service report. The report is an updated version of a previous report.