Senate Majority Leader Mitch McConnell (R-Ky.) says he has lined up the votes to start President Trump’s impeachment trial. But until Speaker Nancy Pelosi (D-Calif.) allows a vote to appoint the House managers to bring over the articles, there can’t be a trial. In the 1999 trial of President Bill Clinton, 13 Republicans were named as House managers, all from the Judiciary Committee.

While we’re waiting, here’s what to expect.

Senators want to supplement the existing trial rules

The impeachment trial rules, last modified in 1986, are silent on many important procedural matters, including the day and time at which the president or his counsel must appear to begin the trial; time limits on stages of the trial and debate on any motions; whether witnesses will be called and how they will be heard; and restrictions on what Senate television cameras can show.

To govern all this, once the trial begins, a simple majority of the Senate can adopt supplementary rules — including the currently contentious question of the timing of motions to call witnesses, which largely divides senators along party lines. Republicans want the Senate to follow the course set in 1999 for Democratic President Clinton’s impeachment trial. But back then, Senate Majority Leader Trent Lott (R-Miss.) and Minority Leader Thomas A. Daschle (D-S.D.) tried but failed to get their colleagues to agree in advance on supplementary rules.

Here’s what happened. As the trial began, other senators, backed by the leaders, agreed to a set of preliminary rules. Under these rules, the two sides would first present their cases, and senators would have an opportunity to question the House trial managers and the White House lawyers. Then any senator could make a motion to dismiss the articles or to call witnesses. Senators did bring both of those motions. Republicans defeated the motion to dismiss the articles, and they succeeded in calling three witnesses. As provided under the 1999 rules, the witnesses were first deposed; House managers then decided not to bring the witnesses before the Senate.

Today’s Democrats want a guarantee that they can call witnesses. Republicans say they have the votes to follow the 1999 process, which allowed a motion on witnesses after the initial stages of the process.

No one really knows how the chief justice will conduct the trial

The Constitution requires that the chief justice of the United States preside over impeachment trials for a president, meaning that the chief rules on all motions and questions of evidence and procedure, although he can refer a question to the Senate without ruling. Under the 1986 rules, senators must submit their questions — for House managers, the president’s counsel and any witnesses — in writing, to be read by the chief justice. The House managers and president’s counsel may raise objections to a question, leaving the chief justice to determine whether the question is relevant or redundant. This may prove important: The 1999 rules that McConnell wants to follow provide that the House and president’s counsel must limit their presentations to the “argument from the record.” The Senate parliamentarian will assist the chief justice on procedure and precedent.

The House managers and president’s counsel can raise objections at any time. The chief justice will rule on most of them but may leave them for the Senate to decide (without debate). Senators will certainly appeal rulings they dislike; those appeals will be decided by majority vote. House managers are itching to request witnesses; they would like to depose additional witnesses — John Bolton, say — and present new evidence. The chief justice could frame these issues in a way that influences pivotal senators’ votes.

There could be pivotal votes — and senators

There are 53 Republicans, 45 Democrats and two independents. Few believe that the Senate will convict Trump. However, some senators’ votes may be pivotal on key motions.

Consider a motion to dismiss the case, which the 1999 rules permitted. Let’s assume that the Democrats would vote unanimously on any question, including this. The GOP could dismiss the case if they lost no more than two GOP votes, giving them a 51-to-49 majority. If they lost three, the question would be whether the chief justice could cast the tie-breaking vote. During the impeachment trial of President Andrew Johnson in 1868, a Senate majority supported the chief justice when he cast two tie-breaking votes on procedural questions.

What would happen if the chief simply refused to vote to break a tie? The motion would fail. On a motion to call witnesses, a tie vote would kill the motion. In that case, Republicans could afford to lose three Republican votes and would still defeat Democratic attempts to call a witness. We can’t know for sure, though; the chief justice could decide to break a tie and may favor the motion to call witnesses.

All this means that two or three or four Republicans — perhaps Susan Collins of Maine, Lisa Murkowski of Alaska, Mitt Romney of Utah or Cory Gardner of Colorado — could be pivotal on these questions, depending on whether the chief votes and on how unified Democrats remain. One or more of those Republicans could oppose their party on a motion to dismiss — while still voting with their party on a motion to call witnesses.

Party strategies could matter

Party leaders have no official role in a trial. Behind the scenes, leaders can coordinate efforts on key motions. They may also coordinate party tactics on questions for the parties and witnesses and on other matters. In setting the rhetorical tone for their colleagues, they could shape public views of the Senate.

Could Democrats try to call Republican senators as witnesses? In principle, yes, but again, it would require a majority vote. The Senate rules say that a senator may “give his testimony standing in his place.” Such a spectacle would rouse many partisan emotions.

At most stages during the trial, senators are not allowed to engage in colloquies or debate unless every senator votes to allow this. They sit quietly during presentations and the questioning of the parties and witnesses. Under the 1986 rules, the trial is conducted in open session unless the Senate votes to close the doors. If senators deliberate at all, they are likely to do so behind closed doors.

Yet senators are unlikely to remain entirely silent in public. More likely, the parties’ public relations staffs, working for the party leaders, will work to shape coverage and public views of the trial through tweets, corridor comments and so many other ways, as they have been for weeks already.

There could be wild cards

The president’s former national security adviser, John Bolton, says that if the Senate subpoenas him, he is willing to testify, which may yield new evidence about the president’s orders. There could be other wild cards.

Democrats may be hoping that the trial will shift public opinion about the president. But that didn’t happen during the impeachment hearings; nor did it happen during the 1998-1999 Clinton impeachment process, as the Pew Research Center reports. During the Watergate hearings, the public increasingly supported removing President Richard Nixon — but, of course, he resigned before Congress could get to either impeachment or a trial.

Steven S. Smith (@ProfStevenSmith) is professor of political science at Washington University in St. Louis and author of, most recently, “The Senate Syndrome: The Evolution of Procedural Warfare in the Modern U.S. Senate” (University of Oklahoma Press, 2014).