Trump was impeached for abuse of power and obstruction of Congress.

At the time of the call, Trump was withholding $391m in military aid from Ukraine, and conditioned a White House meeting on the probes, according to witnesses. Trump denies any wrongdoing.

The inquiry centred on a call between Trump and Ukraine's president in which Trump asks for a probe into the Bidens. Trump also wanted an inquiry into a conspiracy theory about the 2016 elections.

The impeachment trial against Donald Trump kicked off on Thursday as mangers from the House of Representatives read the formal charges against the United States president in front of the Senate.

The Democratic-led House impeached Trump last month for obstruction of Congress and abuse of power related to his dealings with Ukraine. Trump has repeatedly denied any wrongdoing, labelling the impeachment process a "sham" and "witch-hunt".

House Speaker Nancy Pelosi initially stalled on transmitting the articles, saying she was waiting until there was a guarantee from the Republican-held chamber that a fair trial would be held. She decided to finally move ahead amid mounting pressure from Republicans and some Democrats.

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Meanwhile, Senate Majority Leader Mitch McConnell has said he has the votes needed to pass a resolution laying out the rules begin the trial without addressing the question of witnesses before opening arguments are heard.

Democrats, led by Chuck Schumer in the Senate, have provided a list of four people to be called as witnesses. The Republican majority, led by McConnell, have signalled they want swift proceedings with no witnesses.

Although Trump is expected to be acquitted in a trial, several senators may prove crucial in shaping the upcoming proceedings in the 100-member Senate, where 53 seats are currently held by Republicans and a simple majority will have the final say on all procedures and rules.

In addition to McConnell and Schumer, here are 10 senators to watch:

Republicans

Susan Collins

'Open to witnesses'

Senator Susan Collins is known as a moderate Republican and could be a swing vote on a rules package for the trial.

Collins has said she wants to adhere “as closely as practical" to former President Bill Clinton's impeachment trial, which included closed-door witness testimony, and has been working with a small group of Republican lawmakers to make that happen.

She told the Bangor Daily News in early January that Republicans should be "completely open to calling witnesses".

"I am hopeful that we can reach an agreement on how to proceed with the trial that will allow the opportunity for both the House and the president's counsel if they choose to do so," she said.

Days later, she told reporters: “My position is that there should be a vote on whether or not witnesses should be called".

Collins is up for re-election this year in Maine, where Hilary Clinton won in the 2016 presidential election.

Collins has also criticised McConnell and other Trump allies, telling local media"it is inappropriate ... for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us". She has said she would oppose a motion to dismiss.

Sen. Susan Collins surrounded by reporters as she rushes to a vote at the Capitol in Washington [File: J. Scott Applewhite/AP Photo]

Lisa Murkowski

'Disturbed' by McConnell's approach

Senator Lisa Murkowski is also considered to be a moderate Republican.

The Alaska senator made headlines last month when she said she was "disturbed" when McConnell said he was working in "total coordination" with the White House for the trial.

"To me, it means that we have to take that step back from being hand in glove with the defence, and so I heard what leader McConnell had said, I happened to think that that has further confused the process," she told local media.

US Senator Lisa Murkowski speaks at a news conference [File: Leah Millis/Reuters]

She said she thought House Democrats should have gone to the court to compel witnesses who refused to testify to appear.

However, she, along with Senator Mitt Romney, are reportedly part of the effort lead by Senator Collins to allow witnesses in the trial. She previously told local media she is "curious" about what the testimony of former National Security Advisor John Bolton would contain, but would not "pre-judge" the need for his testimony until the proceedings began.

She has not indicated how she will vote on impeachment, but has said she will vote against a motion to dismiss case. She has voted against Republican leaders before, including the confirmation of Trump's controversial Supreme Court nominee, Brett Kavanaugh.

Lamar Alexander

Trump's actions 'inappropriate' but impeachment a 'mistake'

Tennessee Senator Lamar Alexander is retiring next year. Ahead of the House vote in December, he called Trump's actions "inappropriate", but labelled the impeachment inquiry a "mistake".

Senator Lamar Alexander on Capitol Hill [File: Brendan Smialowski/AFP]

"It's inappropriate for the president to be talking with foreign governments about investigating his political opponents, but impeachment would be a mistake," he said in a statement in October, according to The Associated Press news agency.

Alexander told Politico in January that "it's important" a vote is held on whether or not witnesses can be called during the trial. He added he would need to hear the case to decide if he would vote for witnesses, saying he would “if I needed to. Or I might not. Or I might.”

He has also said he wants the trial to be "fair".

"We have a constitutional responsibility to have a fair trial and be impartial in our decision making and it would help if the two leaders could agree on what the procedure should be," CNN quoted Alexander as saying in December.

He has said he would vote against a motion to dismiss the case.

Rand Paul

'I'd advise Republicans not participate in anything that doesn't allow defence witnesses'

Senator Rand Paul has defended Trump on impeachment, but has also indicated that he may not support a rules package that does not include defence witnesses, including the whistle-blower, whose complaint led to the impeachment inquiry, and Hunter Biden, the son of Joe Biden whose work with a Ukrainian gas company has loomed large over the investigation.

"I'd advise Republicans not to participate in anything that doesn't allow defence witnesses," the Kentucky senator was quoted by the Hill news site as saying in November.

He tweeted on January 14: "I look forward to forcing votes to call Hunter Biden and many more!"

Rand Paul participates in a TV interview outside his office at Russell Senate Office Building on Capitol Hill [File: Alex Wong/Getty Images/AFP]

Paul has called for the name of the whistle-blower to be made public. He has also criticised Democrats for delaying sending the articles of impeachment to the Senate.

"Take Nancy Pelosi (please). I don't know if I can stop laughing long enough to air my grievances with Nancy. Her new plan is great - she is going to punish the president by NOT sending his impeachment to Senate? Next, maybe she'll threaten to NOT send us anymore legislation?" he tweeted last month.

Mitt Romney

'Completely open mind'

Senator Mitt Romney has been no stranger to criticising Trump. He slammed the president for calling on Ukraine and China to investigate former Vice President Joe Biden, who is also a 2020 Democratic presidential frontrunner. Trump's demand for a Ukrainian investigation of Biden by Ukraine is at the centre of the impeachment.

Sen. Mitt Romney, a member of the Senate Foreign Relations Committee, on Capitol Hill [File: J. Scott Applewhite/AP Photo]

Romney is the only Republican senator who has publicly said he would vote in favour of hearing Bolton's testimony, and along with Senators Collins and Murkowski, is working to assure the trial allows for motions to hear witnesses.

He has been tight-lipped on what his final vote on impeachment will be, vowing to keep a "completely open mind" during the Senate trial.

"I am doing my very best to keep an open mind and I'm going to wait to make any comments on evidence until we see all evidence, all the facts are laid out," he told CNBC in October.

Martha McSally

'Take a serious look'

Martha McSally is another vulnerable senator heading into the 2020 election season. She lost her Arizona Senate race in 2018, but was appointed to her current seat after her predecessor resigned. Arizona is shaping up to be a battleground state in the 2020 elections.

Senator Martha McSally during a Banking, Housing and Urban Affairs Committee hearing [File: Erin Scott/Reuters]

Her office said she has not been convinced that Trump should be removed from office after a recording surfaced of her suggesting to Republican activists that the president had not abused his power. She has since pledged to review the facts objectively and to work "closely with the White House" on the process for impeachment.

Americans "want us to take a serious look at this and not have it be just partisan bickering going on", she told local media.

Cory Gardner

'Political circus'

Senator Cory Gardner has been more cautious than many of his Republican colleagues when talking about impeachment. He is seeking re-election in Colorado, a swing state historically, where recent polling suggests Trump is losing support.

Senator Cory Gardner arrives for a Republican caucus meeting on Capitol Hill [File: Joshua Roberts/Reuters]

A recent poll found that 54 percent of residents in his state support impeachment.

Gardner has criticised the impeachment process, calling it a "political circus", according to local media. But he has been largely silent on whether he would support having witnesses.

He also stayed mum on rumours that he is working with the small group of Republican senators pushing to allow motions for witnesses in the trial.

Democrats

Doug Jones

'Every trial is a pursuit of the truth. That's all I want'

Senator Doug Jones faces re-election this year in the conservative state of Alabama.

Jones has described the charges against Trump as "serious" but said he is keeping an open mind when it comes to the Senate impeachment trial.

Senator Doug Jones speaks at a Senate Banking Committee hearing [File: Andrew Harnik/AP Photo]

In a Washington Post op-ed last week, Jones said he wants to hear from the four witnesses Democrats have said they want to call in the trial.

"If [Trump] chooses to maintain his blockade, however, the Senate needs only a simple bipartisan majority to issue subpoenas for witness testimony and relevant documents. A full, fair and complete trial demands nothing less," Jones wrote.

"Every trial is a pursuit of the truth. That's all I want. It's all each of us should want," he added.

Krysten Sinema

'Approach it with no bias'

Krysten Sinema, a freshman senator from Arizona, has emerged as one of the more moderate Democrats.

She won a Senate seat by a tight margin and has since voted with Republicans on several issues, including the confirmation of William Barr. She also voted against the Green New Deal.

Kyrsten Sinema speaks after being declared the winner in Arizona's US Senate race [File: Rick Scuteri/AP Photo]

Sinema has remained largely tight-lipped about how she plans to vote on impeachment.

"If the Senate receives articles of impeachment, it is our duty to hold a trial and we will do that," she told local media last month. "As a juror, it will be my constitutional duty to approach it with no bias and to listen to the arguments presented by both sides and make a decision."

Joe Manchin

'I am very much torn'

Senator Joe Manchin from the conservative state of West Virginia has voted with Trump more than any other Democratic senators in office.

Manchin was the only Democrat to vote in favour of confirming Kavanaugh's Supreme Court nomination.

He has supported the idea of having witnesses, but told CNN in December he was "torn" on whether he would vote to convict Trump.

Senator Joe Manchin during a hearing of the Senate Foreign Relations Committee [File: Alex Brandon/AP Photo]

"We have a divided country. On the other hand, we have equal branch of governments, responsibilities in the constitution. There are a lot of things at stake here," he said.

"The future of our country. And the future of how we're able to do our business depends on how we handle this," he added. "I'm very much torn on it. I think it weighs on everybody."