When Chief Justice John Roberts arrives on Capitol Hill in coming days to preside over the impeachment trial of President Donald Trump, Roberts will confront a national moment riven by more partisan division than at any time in recent memory.

Let’s set the stage. Roberts is a Chief Justice appointed by a Republican President, leading a conservative majority on a Supreme Court divided sharply along partisan and ideological lines. He will preside over the trial of the current Republican President, impeached by a Democratic majority in the House of Representatives horrified that President Trump withheld military aid in order to pressure Ukraine into investigating his potential political opponent in the 2020 election. The House passed two articles of impeachment over the objection of Republicans who refused even to acknowledge these facts. That includes many, though not all, Republicans in the Senate who seem determined to rubber-stamp President Trump’s acquittal.

This historic moment has entangled Trump and Roberts, for better or for worse. They may come from the same political party, but they are very different people when it comes to how they approach the institutions of democratic government. Roberts takes very seriously his role as leader of the federal judiciary. He has, in his own diplomatic way, pushed back against Trump’s attacks on individual judges, whether based on their ethnicity or the president who appointed them to the bench.

At the same time, Roberts cares deeply about the Supreme Court and wants to ensure that it is seen as an impartial institution—one that is separate from the partisan, political branches of government. Presiding over Trump’s impeachment trial, the Chief Justice will likely want to be seen as rising above partisanship, serving as an impartial arbiter of justice. To that end, Roberts will likely want to avoid letting the trial become a political circus. By contrast, Trump apparently wants it to devolve into a reality TV event as much as possible. This places Roberts in an awkward position, to say the least.

Contrast this with the impeachment trial of President Clinton in the 1990s. After presiding over that trial, the late Chief Justice Rehnquist liked to say, “I did nothing in particular and I did it very well.” By the same token, Chief Justice Roberts may be similarly reluctant to get involved in the detailed push-and-pull of how to proceed with President Trump’s trial. But the disagreement over the rules that will govern  this proceeding is far more severe than anything seen before Clinton’s trial. Unlike Rehnquist, Roberts may simply be unable to avoid stepping into the fray and getting his robe dirty.

This means that, while the Constitution designates the Chief Justice as the presiding officer when a President is impeached, the exact contours of Roberts’s role in this impeachment trial are still unclear. As he enters the Senate chamber in the days ahead, Republicans and Democrats will likely still be in disagreement over the key issue of subpoenaing witnesses and documents, for example. Rulings by Roberts on these and other questions in favor of fairness, however, can be overturned by a Senate majority composed of Republicans. If they do so on this or other critical issues, Republicans run the risk of the American people questioning the legitimacy of the entire trial, tarnishing Roberts in the process (and by extension, the Court he leads).

That raises another key point: the American people will be watching. Roberts, sitting high on the Senate dais as Presiding Officer, will be perceived as responsible for ensuring that this trial is robust and transparent. Perhaps the only other time in recent years that the Supreme Court will have been so dramatically in the public eye is the partisan, brutal battle over the nomination of Justice Brett Kavanaugh. Roberts will want this proceeding to reflect much better on the Supreme Court in the minds of the American public than that spectacle did.

Roberts comes to this impeachment trial very aware that the eyes of history are upon him. He knows that he is the presiding judge over the impeachment of a president, a solemn constitutional process that has only happened twice before in our Nation. He knows that in this terribly partisan moment, his rulings will be under great scrutiny. He will want this trial to be viewed by the American people as thorough, civil, and above all, fair and impartial. That may not be what President Trump or his Republican his allies in the Senate want, but that is what Chief Justice Roberts must deliver.