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In anticipation of today's proceedings, House impeachment manager Adam Schiff has told reporters he would start by laying out the timeline and facts behind the impeachment inquiry.

"We believe we will make an overwhelming case for the president's conviction," Schiff said. "The facts are damning."

Schiff again echoed calls from top Democrats to include witnesses in the Senate trial, but rejected the idea of a witness "swap" between the two parties, exchanging testimony from someone selected by the Democrats for someone chosen by Republicans.

"This isn't some fantasy football trade," Schiff said.

"The American people overwhelmingly want to hear from these witnesses and for a very good reason: they can shed light on the president's misconduct."

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So far, impeachment has played out along party lines, and Trump, a Republican, is expected to be acquitted in the Senate - controlled by his party.

Still, there is speculation that some Republicans could buck their party at some point in this trial, perhaps when it comes to seeking witness testimony.

Here are three senators worth keeping an eye on:

Susan Collins, Maine

She has a reputation as a moderate Republican, espousing support for abortion rights and LGBT issues. In 1999, she broke with her party by voting to acquit Democratic President Bill Clinton in his impeachment trial.

Collins has already voted with Democrats once in Trump’s impeachment, voting to extend the amount of time House managers and Trump’s legal team have to respond to motions.

She will be closely watched in Maine, where she is up for re-election in November and is considered one of the most politically vulnerable Republicans.

Mitt Romney, Utah

The former Republican presidential nominee is a frequent critic of the president and has already said he’d be interested in hearing from witnesses, including former National Security Adviser John Bolton, whose testimony is much desired by Democrats.

Romney might have more leeway to break from his party. He isn’t up for election again until 2024 and represents a state where Trump isn’t very popular, meaning a vote against the president won’t necessarily amount to a political death warrant upon himself.

Lisa Murkowski, Alaska

She has already proven her willingness to break with Trump and side with Democrats on key issues.

Murkowski famously voted "present" in the confirmation hearing of now-Supreme Court Justice Brett Kavanaugh and helped block a repeal of Obamacare by Republicans.

She has two more years until she is up for reelection - and Alaskans are used to her breaking ranks - so she may have the political latitude to vote alongside Democrats again.

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A recap: President Trump is accused of breaking the law by pressuring Ukraine's leader to dig up damaging information on a political rival.

In July, he urged his Ukrainian counterpart to investigate Joe Biden, one of the front-runners to take him on in next year's presidential election. It is illegal to ask foreign entities for help in winning a US election.

Trump was impeached on 18 December on allegations he abused his power and obstructed Congress by withholding documents and witnesses.

The 100-seat Senate is controlled by 53 Republicans, making it highly unlikely that Trump will be voted out of office.

Want to learn more?

A SIMPLE GUIDE: A basic take on what's going on

GO DEEPER: Here's a 100, 300 and 800-word summary of the story

WHAT'S IMPEACHMENT? It's a political process to remove a president

VIEW FROM TRUMP COUNTRY: Hear from residents in Pennsylvania

CONTEXT: Why Ukraine matters to the US

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Chief Justice of the US Supreme Court John Roberts is presiding over the impeachment trial, in accordance with rules laid out in the US constitution.

But that doesn't mean he gets to skip his normal court duties.

After staying up late to oversee the trial until 2am local time, he was back on the bench at 10am this morning to oversee a Supreme Court case on funding for religious schools.

According to US media, he appeared un-fatigued this morning, and asked questions of both sides' lawyers.

It’s a rare day in US history - only the third time an impeachment trial has ever been held.

Yesterday, the Republican-controlled Senate debated the trial's rules but today the arguments begin in earnest.

Democratic impeachment managers from the House of Representatives begin three days of opening statements.

Mr Trump is accused of abuse of power and obstruction of Congress, which he denies.

We’ll keep you updated on all the latest happenings on Capitol Hill.