The impeachment trial of US President Donald Trump is set to begin in earnest on Tuesday as Republicans and Democrats battle over the rules of the proceedings.

With a 53-47 majority, Republicans are expected to ultimately get their preferred rules package. Senate Majority Leader Mitch McConnell initially proposed 48 hours, divided evenly between House managers and Trump's defence, for opening arguments over six days. The opening arguments would be following by 16 hours of questions and answers from Senators before four hours of debate.

The impeachment trial marks a historic moment in the United States, highlighting the deep divisions in the country.

As the trial gets fully under way, here's what you need to know in fewer than 600 words.

What should you expect to see on Tuesday?

The Senate will convene at 1pm local time (18:00 GMT) to begin debate on the rules resolution. Democrats have already promised to propose amendments. That means the debate could go well into the night. But expect the resolution to ultimately pass along party lines.

Why do the rules matter?

The rules ultimately decide the direction and tone of the trial.

In this image from video, Majority Leader Senator Mitch McConnell, speaks as the impeachment trial against President Donald Trump begins in the Senate [Senate Television/AP Photo]

The rules package of former President Bill Clinton was passed with bipartisan support. But that is unlikely this time.

There's also a question of witnesses. Right now, the rules package puts off the question of witnesses until after opening arguments. That has angered Democrats, who will likely propose amendments on Tuesday that would call witnesses or evidence.

Why was Trump impeached?

House Democrats accuse Trump of abusing his power in office by orchestrating a pressure campaign to get Ukraine to investigate former Vice President Joe Biden, a leading Democratic political rival, as well as launch a probe into a debunked conspiracy theory about the 2016 presidential elections.

The House impeachment investigation centred on a whistle-blower complaint in which Trump spoke to Ukrainian President Volodymyr Zelenskyy on the phone and asked for the investigation into Biden.

At the time of the call, the Trump administration was withholding nearly $400m in military assistance to Ukraine.

During the impeachment inquiry, Trump refused to participate and urged current and former administration staffers to do the same.

He was impeached on December 18 for abuse of power and obstruction of Congress.

When will oral arguments begin?

Oral arguments are expected to begin on Wednesday - that's if the Senate adopts the rules resolution on Tuesday.

First up would be the House mangers, followed by Trump's defence team.

Is it Day 1 or 2 of the trial?

Technically, Tuesday marks the second day of the trial. It officially opened last Thursday when Supreme Court Chief Justice John Roberts swore in the senators as jurors.

Who are the House managers?

House managers act as prosecutors. They include:

Adam Schiff, House Intelligence Committee chairman (team lead) Jerrold Nadler, House Judiciary Committee chairman Zoe Lofgren Hakeem Jeffries Val Demings Jason Crow Sylvia Garcia

House Speaker Nancy Pelosi speaking during a news conference to announce impeachment managers on Capitol Hill in Washington [Susan Walsh/AP Photo]

Who's on Trump's defence team?

Pat Cipollone, White House counsel (team lead) Jay Sekulow Kenneth Starr Alan Dershowitz Robert Ray Pam Bondi Pat Philbin Mike Purpura

What's Trump doing during it all?

The obvious expectation is that he will be tweeting, but how much remains to be seen. The president is currently in the Swiss town of Davos for the four-day annual gathering of the World Economic Forum.

What's the expected outcome?

An acquittal is expected.

A two-thirds majority vote is required in the Senate to convict and remove a president from office.

The Senate is currently made up of 53 Republicans, 45 Democrats and two independents who caucus with the Democrats.

Trump is only the third US president to be impeached. No president has ever been removed from office via the impeachment process.