(Bloomberg Opinion) — A presidential impeachment trial in the Senate is one man’s worst nightmare. That man isn’t President Donald Trump, however. He’ll take it in stride. It’s Chief Justice John Roberts, who will have to preside.Roberts has devoted his whole career to trying to keep the Supreme Court from being seen as a partisan body. That started with the famous baseball analogy he offered at his Senate confirmation, according to which the justices are like umpires who call balls and strikes. The comparison is pretty dubious: balls and strikes aren’t infused with controversial moral questions like when life begins and who can marry whom. But Roberts was trying to illustrate his ideal of a justice who stays out of the partisan fray of team spirit.Since then, Roberts has made a number of important rulings perhaps intended to keep the court from appearing too partisan. For example, he broke with his conservative colleagues to save the individual mandate in the landmark Affordable Care Act case (even though, in the same judgment, he gutted the act’s Medicare expansion) and also sided with the liberals in a 5-4 decision that kept a citizenship question off the 2020 census questionnaire.Trump’s impeachment trial is a nightmare for Roberts because it will be very challenging to avoid an appearance of partisanship while he presides. There’s no ducking the job, though. The Constitution specifically requires the chief justice to preside over presidential impeachment, presumably because the framers realized that it would be awkward for the vice president to be in that chair as the senators decided whether to oust the president and thus elevate the vice president.When Chief Justice William Rehnquist presided over President Bill Clinton’s impeachment trial, he managed to stay out of the politics almost entirely. As Rehnquist later said, quoting his beloved Gilbert and Sullivan, he “did nothing in particular and he did it very well.”Roberts was a law clerk for Rehnquist and would doubtless love to emulate this aspect of his experience. The trouble is that times have changed in ways that will make things much tougher for Roberts than they were for his old boss.When Clinton was impeached, the Senate unanimously adopted rules of procedure that were, in the main, fair and balanced. That meant Rehnquist was rarely asked to decide any substantive matters.That sort of Senate bipartisanship would be terrific this time around. But it seems highly unlikely that, in the current atmosphere of extreme partisanship, the senators will manage to reach consensus, much less unanimity, on rules of procedure or evidence for the trial. That means Roberts could be asked regularly to rule on tough procedural issues, such as which witnesses may be called by whom or whether certain evidence (for example, hearsay) can be admitted.Deciding these questions would put Roberts in a difficult spot. Democrats will criticize him as partisan if he issues rulings favorable to Trump. Republicans will excoriate him as a traitor if he rules against the president.The best strategy available to Roberts may be to rely on a quirk of Senate impeachment rules. Under Senate precedent, a majority of the Senate can overrule any procedural or evidentiary decision made by the chief justice.Rather than ruling and subjecting himself to the indignity of being overruled, Roberts could say to the senators that he would like to them to vote on any close question, skipping the step of issuing a decision himself. There is some precedent for this in earlier impeachment trials, albeit mostly when a non-judge was in the chair. This approach would shift any partisanship to the senators and away from Roberts.The catch is that the senators would almost certainly have to vote to agree to vote before Roberts ruled. If the parties vote in lockstep (which is probable but not certain), then Republicans, who enjoy a slight Senate majority, would themselves have to want to rule before Roberts did. In other words, the Republicans would have to agree to keep Roberts away from potentially controversial decisions.Here the game theory gets interesting. Assume Republicans make Roberts rule, instead of voting to decide the issue themselves. If he decides for Trump, they win — because now the ruling seems more judicial and objective and less partisan.If he rules against Trump, the Republicans can overrule the decision. Doing so would make them look partisan, to be sure. They might prefer not to do it. But in the end, the public already knows both parties are partisan in this particular Senate battle. So the Republicans might be willing to overrule Roberts.At the same time, if Roberts suggests to the Republicans that they should vote before he rules on something, that might signal to the Republicans that he might be prepared to rule against Trump on that issue.In light of this complexity, if Roberts is forced to make controversial rulings, he will have little choice but to try to be objective: to call balls and strikes, and hope for the best. It’s not an attractive role. But Roberts is up to it, if anyone is.To contact the author of this story: Noah Feldman at [email protected] contact the editor responsible for this story: Sarah Green Carmichael at [email protected] column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.Noah Feldman is a Bloomberg Opinion columnist. He is a professor of law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His books include “The Three Lives of James Madison: Genius, Partisan, President.” For more articles like this, please visit us at bloomberg.com/opinion©2019 Bloomberg L.P.