Sen. Mitch McConnell supposes that it’s up to him to determine the way President Donald Trump’s impeachment trial will proceed in the Senate.

“Everything I do during this, I’m coordinating with the White House counsel,” he declared last week, and pledged to use his power as majority leader to assure Trump’s triumphant acquittal.

But McConnell is wrong: It is Chief Justice John Roberts, not the majority leader, who will be making all the key decisions.

This is the plain meaning of the Senate’s “Rules of Procedure and Practice” currently in force for the conduct of impeachments. These rules explicitly provide that the chief justice “shall” preside over the trial, that it is the “Presiding Officer” who “shall direct all forms of the proceedings,” and that he may “rule on all questions of evidence including, but not limited to, questions of relevancy, materiality, and redundancy of evidence.”

Roberts’ powers are not unlimited. The rules explicitly grant “any member of the Senate” the right to object to Roberts’ evidentiary decisions, and if the chief justice stands firm in his opinion, a senator may demand “a vote of the Members of the Senate” on “any such question”—with a simple majority sufficient to overturn Roberts’ ruling. Nevertheless, the Senate’s authority is strictly constrained by the rules it has itself established. While it can reverse particular evidentiary rulings, it can’t bar anybody from appearing as a witness. Instead, it is up to the lawyers representing the House and the president to make these critical decisions, with the proviso that “witnesses shall be examined by one person on behalf of the party producing them, and then cross-examined by one person on the other side.”

To further assure a fair trial, the rules also expressly guarantee the president the right to testify on his own behalf. With the chief justice in the chair, Trump will be in a position to make his case, but not to pontificate, since the House lawyers can cross-examine him. The Rules also explicitly consider a scenario in which Trump boycotts the entire proceeding and refuses even to allow a legal team to represent him. In this case, “the trial shall proceed, nevertheless, as upon a plea of not guilty”—and the Senate should presume the president innocent unless the House can provide compelling evidence of his guilt.

What is more, “the doors of the Senate shall be kept open” so that the nation at large can come to its own decision on the president’s guilt or innocence. It is only when, after hearing all the evidence, senators retire to engage in jury-style deliberations that they can close their doors to the public.

All of these rules were framed to organize the trial of Andrew Johnson in 1868, with leaders like Sen. John Sherman of Ohio emphasizing that the chief justice “is made by the Constitution the presiding officer of this tribunal, and we cannot deprive him of this power.” But these provisions should not be viewed as historical curiosities. They have been reenacted by the Senate as recently as 1986.

Nevertheless, they are not written in stone. It would be perfectly constitutional for the Senate to waive the rules by unanimous consent and organize the trial along different lines—as happened in the Clinton impeachment. It would also be legal if McConnell persuaded enough Republicans to push through sweeping changes on the basis of a bare majority vote.

But if McConnell wants to change the rules, he had better move fast. If the House votes to impeach the president this week, he will be under a compelling obligation to follow the existing rules in their current form. To understand why, consider that these rules begin by requiring the House to “immediately” present its articles of impeachment to the Senate, which “shall, at 1 o’clock afternoon of the day (Sunday excepted) following such presentation … continue in session from day to day … until final judgment shall be rendered.” This means that once the House presents its articles to the Senate, McConnell has less than 24 hours to change the rules in any way that can win a Senate majority—since, at that point, the current rules command that the “Chief Justice shall be administered the oath [“to do impartial justice”] and shall preside over the Senate during … the trial of the person impeached.”

Once John Roberts replaces Vice President Mike Pence as the Senate’s presiding officer, McConnell’s attempt to change the rules would generate a constitutional crisis. As I have noted, the rules explicitly give Roberts, and nobody else, the power to “direct all forms of the proceedings.” If McConnell tried to seize control, Roberts could refuse to allow the Senate to vote on his initiative, especially if McConnell proposed rule changes that were inconsistent with Roberts’ pledge “to do impartial justice.”

After all, the Constitution requires the chief justice “to preside” over the trial, shifting this authority away from the vice president, whom it explicitly names as the Senate’s normal presiding officer. If the founders supposed that the chief justice would play a merely ceremonial role, they would not have stripped the vice president of control in the first place.

At the present moment, the House has not even impeached Trump yet, and so the time is not yet ripe for Roberts to replace Pence in the chair. In the meantime, McConnell is exploiting the vacuum for his own self-aggrandizement. Nevertheless, once Roberts takes over the gavel “immediately” after the bill of impeachment passes over to the Senate, I have no doubt that he will resist any effort to transform the rules’ demand for an “impartial” trial into a partisan power play.

The chief justice is a serious jurist, dedicated to sustaining the Supreme Court’s central position in our system of checks and balances. His impartial conduct of the trial is especially crucial in the aftermath of the blatant partisanship displayed by McConnell and the Senate during the confirmation battle over Brett Kavanaugh. With this episode vividly in the public mind, it is imperative for Roberts to demonstrate, by his actions, that he takes the Constitution seriously and is not merely serving as a pawn in McConnell’s scheme to guarantee an acquittal.

If the majority leader did make an effort to change the rules midstream, this would serve as Roberts’ moment of truth: Will he demonstrate to the tens of millions of viewers that he is determined to put the Constitution above bitter partisan conflict?

Given Roberts’ repeated efforts to sustain the court’s legitimacy in the past, there is every reason to expect him to stand his ground and refuse to allow McConnell’s motion to be considered on the floor. If McConnell continued to defy Roberts and insisted that his colleagues back him up, it seems highly unlikely that his fellow Republicans would provide him with the bare majority needed to provide appropriate window dressing for his attempted constitutional coup. Sens. Susan Collins, Lisa Murkowski, and Mitt Romney have already recognized that the charges against Trump require serious investigation if they reach the Senate. It is hard to believe that they would not heed Roberts’ demand that they act like impartial jurors and refuse to follow McConnell’s commands.

This means that the Senate proceedings will be a very different drama from the repetitive legalisms offered up at the House Judiciary Committee hearings. Instead, the chief justice will almost certainly support the House managers’ demand that administration officials, who have thus far refused to testify, appear as witnesses before the Senate. It is only in this way that the Senate will have all the relevant facts necessary for rendering an impartial verdict. Once they appear in the chamber, they will be interrogated on the crucial issues by lawyers representing the president and those representing the House.

This won’t be the end of the story. After the House managers make their case, it will be the president’s turn to call his witnesses—including, possibly, Joe and Hunter Biden—to refute the charges against him. The Bidens cannot avoid systematic questioning any more than Rudy Giuliani or the other Trump confidantes granted “absolute immunity” by the president. It is only then that the senators will come to a final decision. At that point, McConnell will join all his other colleagues to render their verdict after swearing to “do impartial justice according to the Constitution and laws: So help me God”—to quote the rules one last time.

At this point, we can’t know whether the Senate’s verdict will help or hurt the president’s reelection chances. But if we look ahead a decade or so, John Roberts’ conduct of the trial may well be remembered as a defining constitutional moment in this ongoing tragedy of American democracy.