The US Senate is holding an impeachment trial on charges by the House of Representatives that President Donald Trump abused his powers and obstructed a congressional inquiry into whether he put pressure on Ukraine to investigate Democratic presidential contender Joe Biden.

The Republican-controlled Senate is expected to vote as soon as Friday over whether witnesses and new documents should be part of the trial.

There are 53 Republican senators, 45 Democrats and two independents aligned with the Democrats. If the 47 senators in the minority were to stick together in favour of witnesses, it would take only four Republicans to join them for the effort to succeed.

Democrats want to hear from witnesses who could have first-hand knowledge of Trump's actions, especially his former national security adviser, John Bolton.

Here are the key Republican senators that may swing the balance:

Lisa Murkowski

Republican Lisa Murkowski of Alaska, considered a moderate, says she is working to ensure senators can seek more information, including possibly witnesses and documents.

Although she voted against amendments demanding additional evidence at the trial's outset, Murkowski told reporters in her home state that did not mean that she would oppose a call for witnesses later.

US Senator Lisa Murkowski [File: Leah Millis/Reuters]

First elected to the Senate in 2002, Murkowski, 62, voted against advancing Trump's nomination of Brett Kavanaugh to serve on the Supreme Court in 2018. In 2017, she bucked Trump and most fellow Republicans by voting against the repeal of key elements of the Affordable Care Act, popularly known as Obamacare.

Mitt Romney

Mitt Romney, the 2012 Republican presidential nominee, has said he would be "very likely" to support witness testimony.

The freshman senator from Utah has been one of the few Republicans in Congress to criticise Trump for urging Ukraine and China to investigate Biden, a leading candidate to run against Trump in November's election.

Senator Mitt Romney [File: J Scott Applewhite/The Associated Press]

Trump, in turn, has called the senator a "pompous a**". Romney, 72, says he wants to hear testimony from Bolton.

Susan Collins

Susan Collins of Maine, a centrist Republican, says it is important for senators in both parties to "render impartial justice" during the trial. She has criticised her party's leader in the Senate, Mitch McConnell, for saying he was coordinating trial preparations with the White House.

Senator Susan Collins [File: J Scott Applewhite/The Associated Press]

Collins, 67, says she would likely support a motion to call witnesses once both sides have made opening arguments and senators have had a chance to ask questions. She says she has not decided on any particular witnesses.

Collins has served in the Senate since 1997 and faces a tough re-election race this year in a state divided between rural areas that back Trump and a more liberal region around the city of Portland.

Collins has stood up to Trump at times, voting against the Obamacare repeal in 2017. She voted for Kavanaugh's nomination.

Lamar Alexander

Lamar Alexander, a moderate Republican who is retiring at the end of the year, said he would wait until senators have used up their 16 hours of allotted time to ask questions before making up his mind on whether to include witnesses.

Senator Lamar Alexander [File: Brendan Smialowski/AFP]

He has said it was "inappropriate" for Trump to talk with foreign governments about investigating political opponents but said last year that the president's actions did not warrant impeachment.

Alexander, 79, has been in the Senate since 2003. He has worked with Democrats on some issues, including healthcare. A former governor of Tennessee, he made two unsuccessful runs for president in 1996 and 2000.