It’s official: A Senate impeachment trial is happening, now that the House has adopted its articles of impeachment. While the prospect of a trial has been looming over the Senate for several weeks, given the inevitability of a House impeachment of President Trump’s once the impeachment inquiry was announced in September, many major procedural questions about that trial remain unresolved.

The trial will almost certainly consume Washington, as other work in the upper chamber will come to a screeching halt until after senators decide whether to acquit or convict Trump for his Ukraine pressure campaign. The Senate’s 2020 Democrats will be called back from the presidential campaign trail to sit in on the proceedings, which senators will watch in silence.

The internal debate over what exactly comes next has spilled out into the public in recent days.

Democrats, led by Sen. Chuck Schumer (D-NY), are calling for an agreement before the trial starts that certain key witnesses from the administration be called to testify.

But Majority Leader Mitch McConnell (R-KY), for several reasons, wants to put off such an agreement until the trial is well underway. He’s fending off not just Democratic demands, but also President Trump’s desire for a longer, showier trial, as well as the wishes of a handful of Senate Republicans that they give a fair hearing of the allegations.

Speaker Nancy Pelosi (D-CA) added more uncertainty into the mix Wednesday night when she hinted she might hold off on naming House impeachment managers or transmitting the articles until she has a better sense of what the Senate procedures will look like.

Here’s what we know and don’t know about how the Senate impeachment trial will go down:

Setting Procedures

A Senate impeachment trial changes the power dynamics a bit in the upper chamber. Any request — made by either the House managers or President Trump — for a procedural move can be approved by just 51 members of the Senate. And McConnell doesn’t have full control over what procedural moves are voted on. That means if House impeachment managers and Democrats can find four Senate Republicans to join them, they can approve of procedural moves that are against McConnell’s wishes. Likewise, Trump and McConnell can only lose the support of two members of the GOP caucus if they want approval for procedural moves they desire.

McConnell has emphasized his desire to go the route of the 1999 Clinton trial, in which 100 senators came to an unanimous agreement — hatched in the old Senate chamber — dictating how the first stage, but just the first stage, of the proceedings would go.

The Start Of The Impeachment Trial

All signs point to the Senate trial starting pretty quickly once Congress returns from Christmas break in January — assuming the House transmits the articles by then. Before Wednesday’s vote, House leaders were signaling that any delay in transmission would be minor — and related specifically to legislative reasons. However, Pelosi after Wednesday’s vote wouldn’t make any guarantee about when the articles would be transmitted and suggested she wanted a better idea of what the Senate was planning for the trial before she decided that timing.

Schumer, in a letter Sunday to McConnell, proposed that the Senate adopt some pre-trial housekeeping measures on Monday, Jan. 6, and that the Senate and Supreme Court Chief Justice John Roberts be sworn in on that Tuesday Jan. 7.

If the model of 1999 is followed, there would also a period early on for Trump and the House to submit written briefs before the live action in front of the Senate begins.

McConnell has said he does not expect any difficulty in coming to an agreement for the early phase of the proceedings. But he has rejected Schumer’s request that the witnesses be decided in this initial agreement. Instead, McConnell, citing how the 1999 impeachment unfolded, wants that pre-trial agreement to lay out the procedures for this the first stage of the trial: the opening arguments, senators’ questions, and a motion to dismiss vote.

Putting that disagreement aside, Schumer has proposed that the House managers beginning their opening presentation on Thursday, Jan. 9. After that presentation, the President’s lawyers — expected to be led by White House Counsel Pat Cipollone — would give their opening arguments.

If the Senate follows the practices of the Clinton impeachment proceedings, expect it to meet every day Monday through Saturday at 12:30 or 1 (so that Roberts can attend to Supreme Court business in the mornings) and work for about six hours or longer each day.

Under Schumer’s proposal, the House and President would each get up to 24 hours for their opening presentations, meaning that the preliminary proceedings and opening presentations alone could stretch out two weeks.

Witnesses

The biggest uncertainty about how the Senate trial will unfold is whether witnesses will be called to testify. McConnell is firmly against the idea, pushing instead for a short trial. But it’s unclear if he has 50 other votes in his caucus supporting the no-witness route.

Schumer specifically wants testimony from acting White House Chief of Staff Mick Mulvaney, Mulvaney’s senior advisor Robert Blair, former National Security Advisor John Bolton and Michael Duffey, a top official at the Office of Management and Budget.

Democrats hope they can get the support of enough Republicans to approve witness testimony for the trial. But so far, even the handful Republicans who seem like they’d be most interested bringing in witnesses have fallen in line behind McConnell’s plan to put off the witness decision until midway through the trial.

Under Schumer’s proposal, each witness would be questioned for eight hours (four for each side), and then the trial would proceed to the written questions submitted by senators. Sixteen hours — divided equally between questions for the House managers and Trump — would be devoted to those questions, under Schumer’s proposal.

Ending The Trial

Once the Senate gets through its impasse over witnesses, and has finished the time allotted for written questions submitted by senators, the next expected step is closing arguments from the House impeachment managers and Trump’s team. After that would be deliberations. Whether those deliberations happened behind closed doors or out in the open are another procedural matter the Senate will have to decide.

Once the Senate votes whether to convict or acquit, a two-thirds majority — or 67 votes — is required to remove the President.