A variety of pundits and law professors have suggested that Chief Justice John Roberts will have the opportunity and duty to prevent what they fear will be an impeachment trial skewed unfairly by the Senate Republican majority. They are almost surely in for disappointment, because the Constitution gives the Chief Justice no greater role in the impeachment trial for the President than the Vice President has in other impeachment trials—or indeed for the general legislative business of the Senate. That role is limited to presiding over the Senate. He thus can recognize Senators and make rulings on points of disputes over the Senate’s rules of impeachment. In doing so he must be guided by the Senate’s precedents, not the Court’s, and he can be overruled in these rulings by a Senate majority.

The Constitution is clear that the Chief Justice’s role is that of a presiding officer over a legislature, not some that of some special judicial umpire imported for presidential impeachments. Article I, section 3, clause 6 provides:

The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried the Chief Justice shall preside.

This provision simply makes the Chief Justice the presiding officer for the purposes of presidential impeachment just as Section 3, clause 4 makes the Vice President the presiding officer in other trials of impeachment, as well as for all other Senate proceedings.

Moreover, the reason for substituting the Chief Justice for the Vice President is a simple one. The Vice President would have a clear conflict of interest in presiding over the trial of the President, because he would assume the Presidency upon the President’s conviction. While it is true that the Constitution authorizes the Senate to choose a President Pro Tempore to substitute for the Vice President, it would not be wise to make him the presiding officer. After all, sometimes the Vice President who had assumed the Presidency might be subject to impeachment and the President Pro Tempore might stand to assume the Presidency by statute. That is, in fact, the exact situation of the impeachment trial of Andrew Johnson, in which the Senate Pro Tempore of the time, Benjamin Wade, would have ascended to the Presidency upon Johnson’s impeachment. (Wade voted to convict Johnson amidst some criticism of his conflict, but it would have been a far greater scandal had he not only voted but also presided).

Finally, it is notable that the Constitution does not cross-reference this duty of the Chief Justice in Article III, suggesting again that he is acting as a legislative officer and not as a justice. To the argument that this mixing of legislative and judicial duties offends the separation of powers, there are two answers. First, the branches are not completely separated in the Constitution. For instance, although he is an executive officer, the President acts in the legislative process through the veto. Second, the argument of necessity is compelling. It is not clear where else to lodge the duty of presiding over the impeachment of the President. The Speaker of the House or any of its other members is inappropriate since the House is effectively the prosecutor. And members of the executive department, like cabinet officials, serve under the supervision of the President, the defendant.

If the Chief Justice is just the presiding officer, his power is quite limited under Senate rules and precedent, as Vice Presidents have found, sometimes to their regret. Presiding officers have the power of recognition, although even this power is tempered by the duty to give primacy in most circumstances to recognizing the majority leader. And they can make rulings from the chair on points of order about Senate rules, although the Senate as a whole and not the presiding officer, makes these rules.

But these rulings are subject to the appeal of the full Senate and they can be overruled by majority vote. Recently, the Senate majority under both Republicans and Democrats have used this appeals process to rewrite even the plain meaning of the rules as when they eliminated the sixty vote filibuster first for lower court judges and executive branch officials (a Democratic initiative), and then for the Supreme Court justices (a Republican initiative).

It is true that the Chief Justice might be in a better position to makes his rulings stick than the Vice President not because of any difference in his formal legal position but because of the better reputation of the Supreme Court and the Chief Justice than elected politicians. That reputation is a double-edged sword. The Chief Justice will be chary of making rulings that will put his—and thereby the Court’s—reputation at risk. Thus, expect him to follow the lead of the Senate parliamentarian—a bureaucratic expert in the rules appointed by the Senate majority leader—rather than innovate on his own. Chief Justice Rehnquist praised his own performance during the Bill Clinton impeachment by quoting a fictional lord in Gilbert and Sullivan’s Iolanthe: “I did nothing in particular and did it very well.” The Chief Justice is a former clerk to Rehnquist. Look for him to tread lightly in his predecessor’s footsteps.

One interesting question is whether the Chief Justice, like the Vice President, has a casting vote, should the Senate tie on some motion at the impeachment trial. The casting vote is not relevant to the ultimate vote on conviction or acquittal, because the Constitution requires a two-thirds vote for conviction. Nor it is likely relevant on appeals from the Chief’s rulings: a tie vote would not reverse them. But is possible that the Senate will vote on a non-dispositive motion at some point in the trial.

The Constitution expressly provides that the Vice President has a casting vote, but it does not provide the same for the Chief Justice explicitly. It does say that the Chief Justice is to preside over the Senate, replicating the same function as that of the Vice-President. Moreover, the Vice President would have a casting vote on relevant motions during an impeachment trial and it does not make much sense to structure voting in presidential impeachment differently.

Finally, there is precedent on the matter, not judicial but senatorial. In presiding over the Johnson impeachment trial, Chief Justice Salmon Chase did vote on tie motions. The Senate accepted these votes. I would think that Chief John Roberts would adopt this precedent as resolving the issue even as he surely hopes no such casting voting will ever be required of him. He no doubt enjoys being the median justice on the Court, often determining the outcome in 5-4 decisions. But being the decisive vote in the Senate would make him seem like another politician—an appearance that no modern Supreme Court justice wants to suggest.