A trial without witnesses, congresswoman Garcia continues, would not serve a real purpose.

"It will be no exoneration. It will be no vindication. Not for the president, not for this chamber, and not for the American people."

Recent polling shows many Americans do want the trial to allow witnesses.

Yesterday, a YouGovUS poll found 49% support admitting new evidence in the trial, and 44% believe the Senate is not going to conduct a fair, unbiased trial.

A Quinnipac University poll on Tuesday found 75% of voters are in favour of witnesses and 53% believe the president is not telling the truth about the Ukraine matter.

Conservatives - including Trump's son, Donald Jr - have taken to Twitter to laud Lisa Murkowski for saying she'll vote against witnesses.

Some have also criticised Mitt Romney for doing the opposite.

"We know that there are more relevant documents, there is no dispute about that," says House impeachment manager Sylvia Garcia.

"Witnesses have testified in exceptional detail about these documents that exist that the president is simply hiding."

We cannot let this president be different, she adds. The stakes are too high.

"The Senate requires and should want a complete evidentiary record before you vote on the most sacred task that the constitution entrusts in every single one of you.

"It should not be about party affiliation, it should be about seeing all the evidence and voting your conscience based on al the relevant facts. It should be about doing impartial justice."

She adds: "How can the American people have confidence in the result of a trial without witnesses?"

Image source, Getty Images

Alaska Senator Lisa Murkowski has tweeted her full statement on why she's voting "no" to witnesses.

"I worked for a fair, honest, and transparent process, modeled after the [Bill] Clinton trial, to provide ample time for both sides to present their cases, ask thoughtful questions, and determine whether we need more," she says.

"The House chose to send articles of impeachment that are rushed and flawed. I carefully considered the need for additional witnesses and documents, to cure the shortcomings of its process, but ultimately decided that I will vote against considering motions to subpoena.

"Given the partisan nature of this impeachment from the very beginning and throughout, I have come to the conclusion that there will be no fair trial in the Senate. I don’t believe the continuation of this process will change anything."

Image source, Getty Images

The debate over witnesses grew more intense this week amid reports that former National Security Adviser John Bolton's upcoming book included a claim that Trump linked freezing military aid to Ukraine in exchange for a political favour.

If true, it's significant because Republicans have been arguing that no first-hand witnesses have connected the president to the alleged scheme to withhold aid for political benefit.

On Friday, the New York Times published more details from an unpublished manuscript of Bolton's book. Bolton reportedly writes that Trump directed him to help with the pressure campaign to get dirt on Democratic rivals from Ukraine.

The president reportedly told Bolton to call Ukraine's president Volodymyr Zelensky to ensure a meeting took place between Zelensky and Trump's personal lawyer, Rudy Giuliani.

But in the last few minutes Trump denied these allegations after the article was published, saying he never instructed Bolton to arrange that meeting.

"That meeting never happened," Trump said.

It's a complicated story but you can pick which parts of the tale you want to know more about.

WHAT HAPPENED THIS WEEK: Here's a recap on the trial so far

FIVE POSSIBLE TWISTS: Think it's a foregone conclusion? Perhaps not

JOHN BOLTON: Why does everyone want to talk to him?

SIMPLE GUIDE: If you want a basic take on what's going on

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

IMPEACHMENT QUESTIONS? We’ve got answers

CONTEXT: Why Ukraine matters to the US

Currently, the Democratic impeachment managers are making their final case for witnesses before the Senate will vote on the matter.

Democrats have been arguing that the trial cannot be fair if witnesses are not called.

They say there is precedent that allows for the upper chamber, the Senate, to examine new documents and witnesses that the House of Representatives was unable to access.

We expect the debate to last four hours.

Democrats need four Republicans to vote with them in favour of witnesses - an outcome that seemed possible earlier this week, but now appears unlikely.

There are four moderate Republicans who've been closely watched for their decision on witnesses:

• Lisa Murkowski of Alaska
• Lamar Alexander of Tennessee
• Susan Collins of Maine
• Mitt Romney of Utah

Romney and Collins have said they'd be in favour of new documents and witnesses, but late on Thursday, Alexander said while he thinks the president's actions were inappropraite, they were not impeachable and he would vote no.

On Friday, as the trial kicked off, Murkowski also announced she'll vote no, saying: "I don’t believe the continuation of this process will change anything."

Image source, Getty Images

Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office.

When the House of Representatives votes to pass articles of impeachment, the Senate must then hold a trial.

A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Trump's party controls the chamber.

Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed.

President Nixon resigned before he could have been impeached.

The Senate impeachment trial of President Donald Trump - the third-ever such trial in history - has moved from written questions into a debate over whether new witnesses will be allowed.

Democrats have been calling for witnesses, including former national security adviser John Bolton, to testify in light of new reports - but they need four Republicans to join them to win, as Republicans have a majority in the Senate.

Stay tuned for our live coverage as this next phase of the trial unfolds.