Up to now, the Senate’s impeachment trial of President Donald Trump has unfolded along party lines. All but one of the votes that senators cast earlier this week  on efforts by Minority Leader Chuck Schumer, D-NY, to change the supplemental rules package (S. Res. 483 ) proposed by Majority Leader Mitch McConnell, R-Ky., were decided on a party-line basis (53 Republicans opposing 47 Democrats). And Republican senators like Susan Collins, R-Maine, Lisa Murkowski, R-Alaska, and Mitt Romney, R-Utah, have not yet signaled that they will break ranks on procedural questions in the future.

But what would happen if three Republican senators joined all 47 Democrats in support of a motion to call a witness or demand that the administration produce additional documents? In that scenario, the result would be a tie vote (50-50).

The underlying question is defeated when senators are evenly divided because it lacks a majority to pass. However, the Constitution and the Senate’s rules and precedents also empower the Vice President and the Chief Justice, respectively, to cast tie-breaking votes in certain circumstances.

The Constitution

The Constitution empowers the Vice President to cast tie-breaking votes. Specifically, Article I, section 3, clause 4  stipulates, “The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided.”

The Constitution does not empower the Chief Justice to cast tie-breaking votes. Article I, section 3, clause 6  stipulates only that “When the President of the United States is tried, the Chief Justice shall preside.”

In other words, the Constitution designates the Vice President the President of the Senate and  gives the Vice President the power to cast tie-breaking votes. Yet the Constitution only designates the Chief Justice the presiding officer in presidential impeachment trials. The Constitution does not designate the Chief Justice the President of the Senate, nor does it give the Chief Justice the power to cast tie-breaking votes. This suggests that the Vice President retains the power to cast tie-breaking votes in presidential impeachment trials.

The Senate’s rules and precedents

The Senate’s rules and precedents permit the Chief Justice to cast tie-breaking votes when presiding in presidential impeachment trials. In 1868, Chief Justice Salmon P. Chase cast two tie-breaking votes while presiding in President Andrew Johnson’s impeachment trial. (The Chief Justice declined to vote on a third occasion later in the trial.) At the time, senators approved, albeit implicitly, the actions of the Chief Justice when they refrained from reversing them on appeal.

While the Constitution does not give the Chief Justice the power to cast tie-breaking votes in presidential impeachment trials, it does give the Senate plenary power over its rules of procedure. Specifically, Article I, section 5, clause 2  (the Rules and Expulsion Clause) stipulates, “Each House [of Congress ] may determine the Rules of its Proceedings.” The Senate uses this authority to sanction its rules and precedents which, by extension, permit the Chief Justice to cast tie-breaking votes while presiding in presidential impeachment trials.

The Supreme Court

The Supreme Court ruled in 1892 that under the Rules and Expulsion Clause, absent an explicit constitutional provision stipulating otherwise, the House and Senate are free to make any rules they choose using their plenary power to determine their rules of procedure. Writing for the Court in United States v. Ballin , Justice David Brewer noted that while “the Constitution empowers each house to determine its rules of proceedings,” by their rules, the House and Senate could not “ignore constitutional restraints or violate fundamental rights.” Aside from this exception, the Court held that the power to make rules is exercised by a majority of each chamber and cannot be limited by any rule other than those provided in the Constitution.

According to the logic of the Court’s decision in Ballin , the only requirement to permit the Chief Justice to cast tie-breaking votes while presiding in presidential impeachment trials is “the presence of a majority.” And the Senate cannot use its rule-making power to prohibit the Vice President from casting a tie-breaking vote whenever the Senate is evenly divided because that power is guaranteed by Article I, Section 3, clause 4.

The chief justice vs. the vice president

Under the Constitution, the Senate must be “equally divided” before the Vice President can vote. The Senate is not equally divided if the Chief Justice exercises his power under the Senate’s rules and precedents to cast a tie-breaking vote while presiding in a presidential impeachment trial. The Constitutional directive to preside places the Chief Justice ahead of the Vice President in the sequence of events that must unfold when the Senate votes. And the Senate’s rule-making power empowers the Chief Justice to cast a tie-breaking vote at that moment if the Senate is evenly divided. If the Chief Justice casts a tie-breaking vote in this scenario, the result is that the Senate is not evenly divided and the Vice President is not allowed to vote.

If, however, the Chief Justice declines to cast a tie-breaking vote and instead announces that the Senate is evenly divided, the Vice President is empowered to vote (or not vote). The form specified in the Senate’s precedents that the Vice President uses to vote is compatible with this scenario. “The Senate being equally divided, the Vice President votes in the ___ and the proposition is ___.” The Vice President could also say, “The Senate being equally divided, the Vice President declares the proposition lost or disagreed to.” Note that the Vice President does not reference his role as presiding officer in either example to justify his power to cast a tie-breaking vote.

Implications

Two things would have to happen for a scenario to arise in which Vice President Mike Pence casts a tie-breaking vote in Trump’s impeachment trial.

First, the Senate would have to be evenly divided on a question that the Vice President supports. Questions that the Vice President opposes are defeated when the Senate is evenly divided. The Vice President’s preferred outcome prevails in such circumstances even if he does not vote.

Second, the Chief Justice must decline to cast a tie-breaking vote, thereby ensuring that the Senate is evenly divided at the point when the Vice President is authorized to act under the Constitution.

What happens now?

Of course, the Senate would be in unchartered territory if the scenario above played out as described. Senators do not appear to have adjudicated this question in the past. And the decision to do so now, during Trump’s impeachment trial, would likely destabilize the chamber and increase the chances that unanticipated outcomes would result.