U.S. President Donald Trump's impeachment trial is set to begin in the Senate, but with few precedents and limited specific guidance the process ahead is far from certain.

There are two charges against Trump: Abuse of power by pressuring Ukraine to probe former Vice President Joe Biden and obstructing Congress' efforts to investigate.

The articles of impeachment have been delivered to the Senate, but debate continues as to the rules by which the trial will be run as well as the possibility of new evidence and witness testimony.

Here are the likely next steps:

Senators sworn in

John Roberts, the Supreme Court chief justice who will preside over the trial, will vow to administer "impartial justice" before swearing in each senator as, effectively, jurors.

Each senator must take an oath: "I solemnly swear that in all things appertaining to the trial of the impeachment of Donald Trump, now pending, I will do impartial justice according to the constitution and laws, so help me god."

The prosecution

Seven impeachment managers, all Democratic members of the House of Representatives, will prosecute the case for conviction.

The team led by Adam Schiff, the chair of the House intelligence committee, is completed by Jerrold Nadler, Hakeem Jeffries, Zoe Lofgren, Jason Crow, Val Demings and Sylvia Garcia.

The defense

The defense is expected to be led by White House counsel Pat Cipollone with Trump's personal lawyer Jay Sekulow also playing a central role.

Other lawyers, potentially including Cipollone deputies Pat Philbin and Mike Purpura, are likely to contribute. There is also a possibility that high profile Republican Trump supporters in the House, including Jim Jordan and Doug Collins, will play a role.

Vote on the rules

The Senate must vote on the rules for the trial – a majority vote – before it can begin.

This is contentious: Democrats insist there must be a provision for witnesses to give testimony, but Senate Majority Leader Mitch McConnell says he has enough votes in the chamber – which Republicans control 53-47 – to start the trial without a guarantee to allow witnesses. 

McConnell has said the rules should be similar to those adopted during the trial of Bill Clinton in 1999 – the initial resolution did not guarantee witnesses, but a follow-up passed in a subsequent vote did.

The most likely outcome, given that several Republicans have voiced support for some witness testimony, is that majority votes on witnesses will be held on a case-by-case basis. Four Republicans would have to vote with the Democrats to agree to hear from a witness like John Bolton, the former national security adviser who has said he would give evidence if subpoenaed.

Trial begins

The defense and prosecution will present opening arguments. 

The impeachment managers will first make their case and the defense will follow. Senators cannot make verbal contributions, but can submit written questions.

Both teams will probably be able to invite or subpoena witnesses, subject to case-by-case votes. 

The trial can be dismissed on a simple majority vote at any stage, though McConnell has suggested he doesn't have the votes to do so at present. 

The trial will run six days a week, with Sunday off.

Vote to convict/acquit

The senators will deliberate before casting two separate votes on the articles of impeachment: Abuse of power and obstruction of Congress.

A two-thirds majority is required to convict Trump, so assuming all senators vote 67 is the magic number. That would probably mean 20 Republicans voting against the president, a highly unlikely outcome. 

Conviction means immediate removal from office and President Mike Pence, acquittal marks the end of the process – and a major talking point on the 2020 campaign trail.