Now that the House has approved articles of impeachment, a Senate trial — where lawmakers will decide whether or not to convict Trump of the two charges he faces — is up next.

The timing of this trial, however, is somewhat uncertain given Speaker Nancy Pelosi’s decision to delay the transmission of articles of impeachment. As Vox’s Andrew Prokop reports, Pelosi announced Wednesday that she intends to hold off on sending the articles to the Senate until she has a clearer sense of the process the upper chamber plans to use for the trial.

Senate rules indicate that it can’t begin the trial until the articles are received, George Washington University political science professor Sarah Binder tells Vox.

Pelosi’s latest strategic maneuver comes as Democratic concerns have grown about the fairness of the trial, following statements from Senate Majority Leader Mitch McConnell about working in lockstep with the White House to conduct it. By holding onto the articles for now, Pelosi appears to think she could potentially pressure the Senate into adopting terms that are perceived as more equitable.

At this point, a Senate conviction of Trump is looking incredibly unlikely: 67 senators would need to vote in favor of it, a move that would require 20 Republicans to join with the 47-member Democratic caucus and break with the president.

The upper chamber’s trial, however, is intended to provide the opportunity for both House lawmakers and Trump to make their case. Unsurprisingly, as Pelosi’s move indicates, lawmakers from both parties currently have different opinions about what the trial should include.

Republicans have suggested that a trial could end in as quickly as two weeks: They’ve said they aren’t particularly interested in calling new witnesses, and are eager to move rapidly to a vote on acquittal. Democrats, meanwhile, would like to call four witnesses who have direct knowledge about the Ukraine scandal, a request McConnell has balked at for now.

The Senate trial was previously expected to kick off 2020, with the potential of starting as early as Monday, January 6. Due to the latest developments, however, it’s unclear whether that timeline will still hold.

When it happens, Trump’s trial would mark just the third time in history the upper chamber has undertaken this process: Both Presidents Bill Clinton and Andrew Johnson were acquitted in the past.

What the Senate does in impeachment

The Senate’s role in an impeachment proceeding is to serve as the “court” where the charges are reviewed and evaluated. Following House approval of articles of impeachment, it will be the Senate’s job to analyze these charges in a trial and decide whether to convict or acquit.

As part of this arrangement, the 100 lawmakers of the Senate will be sworn in as “jurors” who will hear the evidence presented by both sides. Chief Justice John Roberts will preside over the trial, and both sides will have the opportunity to request documents, present arguments and question witnesses if there are any.

On behalf of House lawmakers bringing the impeachment charges, House representatives and their associated counsel are poised to serve as prosecutors, also known as “managers.” Trump’s counsel, meanwhile, will defend him.

According to the Congressional Research Service, there is no set method for presenting evidence. Much like the House approved a resolution outlining the procedures of its impeachment inquiry, the Senate will have to vote on the approach they’d like to take before the trial begins.

This protocol is set to be negotiated by Senate Majority Leader Mitch McConnell and Minority Leader Chuck Schumer. It’s poised to be the first major conflict between the two parties ahead of the trial, especially since both lawmakers have signaled different approaches on the matter.

Even as Schumer has called for the approval of witness testimony upfront, for example, McConnell has argued that counsel should present their arguments first and then lawmakers should weigh the question of witnesses.

During Clinton’s impeachment, a resolution laying out the timing and schedule of the trial was approved unanimously by the Senate, though another motion raised later in the trial about calling witnesses passed solely along party lines.

The ultimate structure of the trial could take a couple different forms. During Clinton’s trial, witnesses did not testify in front of the full Senate. Instead, following the Senate motion on the matter, witnesses participated in depositions, excerpts of which were later shared with lawmakers. Senators will have to decide whether they’re interested in taking a similar approach this time around.

Once the evidence has been presented, senators will deliberate and then hold public votes on each article of impeachment.

The trial is expected to run for six days a week when it’s in session. In a shift from other hearings the Senate holds, lawmakers will be barred from speaking at it, though they will be able to submit written questions to the House managers and defense counsel.

How Senate Republicans plan to mount a defense of Trump

While a trial has yet to get underway, Senate Republicans have offered a glimpse of how they’ll likely try to defend Trump. Already, as Riley Beggin writes for Vox, many lawmakers have indicated that they’ve made up their minds on impeachment.

Rather than confronting the substance of the allegations against him, senators seem intent on redirecting the focus of the trial to former Vice President Joe Biden, who Trump pressured Ukraine to investigate, and other figures like the whistleblower whose complaint helped launch the impeachment inquiry.

These counternarratives don’t rebut the allegations Trump faces so much as they seek to distract from them, as outlined by the New York Times:

That narrative will include claims that Ukrainians meddled in the 2016 election instead of the Russians — an unfounded allegation refuted by the administration’s own intelligence agencies as recently as this week — and that Hunter Biden, the younger son of Vice President Joseph R. Biden Jr., used his father’s connections to make money in Ukraine.

Sen. Lindsey Graham has signaled lawmakers’ commitment to this strategy by requesting documents from the State Department tied to former Vice President Joe Biden’s efforts to fire Ukrainian prosecutor Viktor Shokin. As part of their case, Republicans have argued that Biden’s decision to do so was aimed at shielding his son Hunter Biden, a board member of Burisma, a Ukrainian energy company, though there’s no evidence that suggests this was true.

“If there’s nothing there I’ll be the first one to say there’s nothing there. But we’re not going to live in a country where only one party gets investigated,” Graham told Politico of his decision to more forward with this push.

Sens. Ron Johnson and Chuck Grassley, too, have requested more information about a former Democratic National Committee consultant’s interactions with Ukrainian officials during the 2016 election, a move aimed at legitimizing the disproved conspiracy theory that Democrats colluded with Ukraine in an effort to fix the 2016 election in favor of Hillary Clinton.

In doing so, the senators hope to suggest Trump was focused on investigations in Ukraine in order to protect the sanctity of US elections (and to obscure the fact that the impeachment proceedings largely hinge on allegations Trump tried to pressure Ukraine into meddling in an election). Various lawmakers, including Trump, have also said that the whistleblower who filed the original complaint about Trump’s interactions with Ukraine should be among the witnesses called during the impeachment trial.

Each of these efforts is aimed at shifting the attention from Trump’s request for investigations of Hunter Biden to put the pressure on Democrats. They don’t, however, grapple with the core questions of wrongdoing that have been levied against him, and seek to make an impeachment trial as much about Democrats and the Bidens as it is about Trump.