The Senate seems certain to keep President Donald Trump in office thanks to the overwhelming GOP support expected in his impeachment trial. But how that trial will proceed — and when it will begin — remains to be seen.

Democrats are pushing for the Senate to issue subpoenas for witnesses and documents, pointing to reports that they say have raised new questions about Mr Trump’s decision to withhold military aid from Ukraine.

Once the house transmits the articles of impeachment, decisions about how to conduct the trial will require 51 votes. With Republicans controlling the Senate 53-47, Democrats cannot force subpoenas on their own.

For now, Republicans are holding the line behind Senate majority leader Mitch McConnell‘s position that they should start the trial and hear arguments from House prosecutors and Mr Trump’s defence team before deciding what to do.

But small cracks in GOP unity have appeared, with two Republican senators criticising Mr McConnell’s pledge of “total coordination” with the White House during the impeachment trial.

Trump impeachment: Who's who in the Ukraine scandal Show all 26 1 /26 Trump impeachment: Who's who in the Ukraine scandal Trump impeachment: Who's who in the Ukraine scandal Donald Trump Accused of abusing his office by pressing the Ukrainian president in a July phone call to help dig up dirt on Joe Biden, who may be his Democratic rival in the 2020 election. He also believes that Hillary Clinton’s deleted emails - a key factor in the 2016 election - may be in Ukraine, although it is not clear why. EPA Trump impeachment: Who's who in the Ukraine scandal The Whistleblower Believed to be a CIA agent who spent time at the White House, his complaint was largely based on second and third-hand accounts from worried White House staff. Although this is not unusual for such complaints, Trump and his supporters have seized on it to imply that his information is not reliable. Expected to give evidence to Congress voluntarily and in secret. Getty Trump impeachment: Who's who in the Ukraine scandal The Second Whistleblower The lawyer for the first intelligence whistleblower is also representing a second whistleblower regarding the President's actions. Attorney Mark Zaid said that he and other lawyers on his team are now representing the second person, who is said to work in the intelligence community and has first-hand knowledge that supports claims made by the first whistleblower and has spoken to the intelligence community's inspector general. The second whistleblower has not yet filed their own complaint, but does not need to to be considered an official whistleblower. Getty Trump impeachment: Who's who in the Ukraine scandal Rudy Giuliani Former mayor of New York, whose management of the aftermath of the September 11 attacks in 2001 won him worldwide praise. As Trump’s personal attorney he has been trying to find compromising material about the president’s enemies in Ukraine in what some have termed a “shadow” foreign policy. In a series of eccentric TV appearances he has claimed that the US state department asked him to get involved. Giuliani insists that he is fighting corruption on Trump’s behalf and has called himself a “hero”. AP Trump impeachment: Who's who in the Ukraine scandal Volodymyr Zelensky The newly elected Ukrainian president - a former comic actor best known for playing a man who becomes president by accident - is seen frantically agreeing with Trump in the partial transcript of their July phone call released by the White House. With a Russian-backed insurgency in the east of his country, and the Crimea region seized by Vladimir Putin in 2014, Zelensky will have been eager to please his American counterpart, who had suspended vital military aid before their phone conversation. He says there was no pressure on him from Trump to do him the “favour” he was asked for. Zelensky appeared at an awkward press conference with Trump in New York during the United Nations general assembly, looking particularly uncomfortable when the American suggested he take part in talks with Putin. AFP/Getty Trump impeachment: Who's who in the Ukraine scandal Mike Pence The vice-president was not on the controversial July call to the Ukrainian president but did get a read-out later. However, Trump announced that Pence had had “one or two” phone conversations of a similar nature, dragging him into the crisis. Pence himself denies any knowledge of any wrongdoing and has insisted that there is no issue with Trump’s actions. It has been speculated that Trump involved Pence as an insurance policy - if both are removed from power the presidency would go to Democratic House Speaker Nancy Pelosi, something no Republican would allow. AP Trump impeachment: Who's who in the Ukraine scandal Rick Perry Trump reportedly told a meeting of Republicans that he made the controversial call to the Ukrainian president at the urging of his own energy secretary, Rick Perry, and that he didn’t even want to. The president apparently said that Perry wanted him to talk about liquefied natural gas - although there is no mention of it in the partial transcript of the phone call released by the White House. It is thought that Perry will step down from his role at the end of the year. Getty Trump impeachment: Who's who in the Ukraine scandal Joe Biden The former vice-president is one of the frontrunners to win the Democratic nomination, which would make him Trump’s opponent in the 2020 election. Trump says that Biden pressured Ukraine to sack a prosecutor who was investigating an energy company that Biden’s son Hunter was on the board of, refusing to release US aid until this was done. However, pressure to fire the prosecutor came on a wide front from western countries. It is also believed that the investigation into the company, Burisma, had long been dormant. Reuters Trump impeachment: Who's who in the Ukraine scandal Hunter Biden Joe Biden’s son has been accused of corruption by the president because of his business dealings in Ukraine and China. However, Trump has yet to produce any evidence of corruption and Biden’s lawyer insists he has done nothing wrong. AP Trump impeachment: Who's who in the Ukraine scandal William Barr The attorney-general, who proved his loyalty to Trump with his handling of the Mueller report, was mentioned in the Ukraine call as someone president Volodymyr Zelensky should talk to about following up Trump’s preoccupations with the Biden’s and the Clinton emails. Nancy Pelosi has accused Barr of being part of a “cover-up of a cover-up”. AP Trump impeachment: Who's who in the Ukraine scandal Mike Pompeo The secretary of state initially implied he knew little about the Ukraine phone call - but it later emerged that he was listening in at the time. He has since suggested that asking foreign leaders for favours is simply how international politics works. Gordon Sondland testified that Pompeo was "in the loop" and knew what was happening in Ukraine. Pompeo has been criticised for not standing up for diplomats under his command when they were publicly criticised by the president. AFP via Getty Trump impeachment: Who's who in the Ukraine scandal Nancy Pelosi The Democratic Speaker of the House had long resisted calls from within her own party to back a formal impeachment process against the president, apparently fearing a backlash from voters. On September 24, amid reports of the Ukraine call and the day before the White House released a partial transcript of it, she relented and announced an inquiry, saying: “The president must be held accountable. No one is above the law.” Getty Trump impeachment: Who's who in the Ukraine scandal Adam Schiff Democratic chairman of the House intelligence committee, one of the three committees leading the inquiry. He was criticized by Republicans for giving what he called a “parody” of the Ukraine phone call during a hearing, with Trump and others saying he had been pretending that his damning characterisation was a verbatim reading of the phone call. He has also been criticised for claiming that his committee had had no contact with the whistleblower, only for it to emerge that the intelligence agent had contacted a staff member on the committee for guidance before filing the complaint. The Washington Post awarded Schiff a “four Pinocchios” rating, its worst rating for a dishonest statement. Reuters Trump impeachment: Who's who in the Ukraine scandal Lev Parnas and Igor Fruman Florida-based businessmen and Republican donors Lev Parnas (pictured with Rudy Giuliani) and Igor Fruman were arrested on suspicion of campaign finance violations at Dulles International Airport near Washington DC on 9 October. Separately the Associated Press has reported that they were both involved in efforts to replace the management of Ukraine's gas company, Naftogaz, with new bosses who would steer lucrative contracts towards companies controlled by Trump allies. There is no suggestion of any criminal activity in these efforts. Reuters Trump impeachment: Who's who in the Ukraine scandal William Taylor The most senior US diplomat in Ukraine and the former ambassador there. As one of the first two witnesses in the public impeachment hearings, Taylor dropped an early bombshell by revealing that one of his staff – later identified as diplomat David Holmes – overheard a phone conversation in which Donald Trump could be heard asking about “investigations” the very day after asking the Ukrainian president to investigate his political enemies. Taylor expressed his concern at reported plans to withhold US aid in return for political smears against Trump’s opponents, saying: “It's one thing to try to leverage a meeting in the White House. It's another thing, I thought, to leverage security assistance -- security assistance to a country at war, dependent on both the security assistance and the demonstration of support." Getty Images Trump impeachment: Who's who in the Ukraine scandal George Kent A state department official who appeared alongside William Taylor wearing a bow tie that was later mocked by the president. He accused Rudy Giuliani, Mr Trump’s personal lawyer, of leading a “campaign of lies” against Marie Yovanovitch, who was forced out of her job as US ambassador to Ukraine for apparently standing in the way of efforts to smear Democrats. Getty Images Trump impeachment: Who's who in the Ukraine scandal Marie Yovanovitch One of the most striking witnesses to give evidence at the public hearings, the former US ambassador to Ukraine received a rare round of applause as she left the committee room after testifying. Canadian-born Yovanovitch was attacked on Twitter by Donald Trump while she was actually testifying, giving Democrats the chance to ask her to respond. She said she found the attack “very intimidating”. Trump had already threatened her in his 25 July phone call to the Ukrainian president saying: “She’s going to go through some things.” Yovanovitch said she was “shocked, appalled and devastated” by the threat and by the way she was forced out of her job without explanation. REUTERS Trump impeachment: Who's who in the Ukraine scandal Alexander Vindman A decorated Iraq War veteran and an immigrant from the former Soviet Union, Lt Col Vindman began his evidence with an eye-catching statement about the freedoms America afforded him and his family to speak truth to power without fear of punishment. One of the few witnesses to have actually listened to Trump’s 25 July call with the Ukrainian president, he said he found the conversation so inappropriate that he was compelled to report it to the White House counsel. Trump later mocked him for wearing his military uniform and insisting on being addressed by his rank. Getty Images Trump impeachment: Who's who in the Ukraine scandal Jennifer Williams A state department official acting as a Russia expert for vice-president Mike Pence, Ms Williams also listened in on the 25 July phone call. She testified that she found it “unusual” because it focused on domestic politics in terms of Trump asking a foreign leader to investigate his political opponents. Getty Images Trump impeachment: Who's who in the Ukraine scandal Kurt Volker The former special envoy to Ukraine was one of the few people giving evidence who was on the Republican witness list although what he had to say may not have been too helpful to their cause. He dismissed the idea that Joe Biden had done anything corrupt, a theory spun without evidence by the president and his allies. He said that he thought the US should be supporting Ukraine’s reforms and that the scheme to find dirt on Democrats did not serve the national interest. Getty Images Trump impeachment: Who's who in the Ukraine scandal Tim Morrison An expert on the National Security Council and another witness on the Republican list. He testified that he did not think the president had done anything illegal but admitted that he feared it would create a political storm if it became public. He said he believed the moving the record of the controversial 25 July phone call to a top security server had been an innocent mistake. Getty Images Trump impeachment: Who's who in the Ukraine scandal Gordon Sondland In explosive testimony, one of the men at the centre of the scandal got right to the point in his opening testimony: “Was there a quid pro quo? Yes,” said the US ambassador to the EU who was a prime mover in efforts in Ukraine to link the release of military aid with investigations into the president’s political opponents. He said that everyone knew what was going on, implicating vice-president Mike Pence and secretary of state Mike Pompeo. The effect of his evidence is perhaps best illustrated by the reaction of Mr Trump who went from calling Sondland a “great American” a few weeks earlier to claiming that he barely knew him. AP Trump impeachment: Who's who in the Ukraine scandal Laura Cooper A Pentagon official, Cooper said Ukrainian officials knew that US aid was being withheld before it became public knowledge in August – undermining a Republican argument that there can’t have been a quid pro quo between aid and investigations if the Ukrainians didn’t know that aid was being withheld. Getty Images Trump impeachment: Who's who in the Ukraine scandal David Hale The third most senior official at the state department. Hale testified about the treatment of Marie Yovanovitch and the smear campaign that culminated in her being recalled from her posting as US ambassador to Ukraine. He said: “I believe that she should have been able to stay at post and continue to do the outstanding work.” EPA Trump impeachment: Who's who in the Ukraine scandal Fiona Hill Arguably the most confident and self-possessed of the witnesses in the public hearings phase, the Durham-born former NSC Russia expert began by warning Republicans not to keep repeating Kremlin-backed conspiracy theories. In a distinctive northeastern English accent, Dr Hill went on to describe how she had argued with Gordon Sondland about his interference in Ukraine matters until she realised that while she and her colleagues were focused on national security, Sondland was “being involved in a domestic political errand”. She said: “I did say to him, ‘Ambassador Sondland, Gordon, this is going to blow up’. And here we are.” AP Trump impeachment: Who's who in the Ukraine scandal David Holmes The Ukraine-based diplomat described being in a restaurant in Kiev with Gordon Sondland while the latter phoned Donald Trump. Holmes said he could hear the president on the other end of the line – because his voice was so “loud and distinctive” and because Sondland had to hold the phone away from his ear – asking about the “investigations” and whether the Ukrainian president would cooperate. REUTERS

Alaska senator Lisa Murkowski said she was “disturbed” by the GOP leader’s comments, adding that there should be distance between the White House and the Senate on how the trial is conducted. Maine senator Susan Collins, meanwhile, called the pledge by Mr McConnell inappropriate and said she is open to seeking testimony.

Democrats could find their own unity tested if and when the Senate reaches a final vote on the two house-approved impeachment charges — abuse of power and obstruction of congress.

It would take 67 votes to convict Mr Trump on either charge and remove him from office, a high bar unlikely to be reached. It’s also far from certain that all 47 Democrats will find Mr Trump guilty.

Democratic senator Doug Jones of Alabama said he’s undecided on how he might vote and suggested he sees merits in the arguments both for and against conviction.

A look at senators to watch once the impeachment trial begins:

Lisa Murkowski

In her fourth term representing Alaska, Ms Murkowski is considered a key Senate moderate. She has voted against GOP leadership on multiple occasions and opposed Brett Kavanaugh‘s nomination to the Supreme Court in 2018.

Murkowshi: 'To me it means that we have to take that step back from being hand in glove with the defence' (Getty)

Murkowski told an Alaska TV station last month there should be distance between the White House and the GOP-controlled Senate in how the trial is conducted.

“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 said.

Ms Murkowski says the Senate is being asked to cure deficiencies in the house impeachment effort, particularly when it comes to whether key witnesses should be brought forward to testify, including White House acting chief of staff Mick Mulvaney and former national security adviser John Bolton.

“How we will deal with witnesses remains to be seen,” she said, adding that house leaders should have gone to court if witnesses refused to appear before Congress.

Collins 'It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us' (Getty)

Susan Collins

The four-term senator said she is open to calling witnesses as part of the impeachment trial but calls it “premature” to decide who should be called until evidence is presented.

“It is inappropriate, in my judgment, for senators on either side of the aisle to prejudge the evidence before they have heard what is presented to us,” Ms Collins told Maine Public Radio.

Senators take an oath to render impartial justice during impeachment — an oath lawmakers should take seriously, Ms Collins said.

Ms Collins, who is running for re-election and is considered one of the nation’s most vulnerable GOP senators, also faulted Democrats for saying Mr Trump should be found guilty and removed from office. “There are senators on both sides of the aisle, who, to me, are not giving the appearance of and the reality of judging that’s in an impartial way,” she said.

Senators are 'are headed towards a trial that is not intended to find the whole truth' (Getty)

Doug Jones

Mr Jones, a freshman seeking re-election in staunchly pro-Trump Alabama, is considered the Democrat most likely to side with Republicans in a Senate trial. In a Washington Post op-ed column, Mr Jones said that for Americans to have confidence in the impeachment process, “the Senate must conduct a full, fair and complete trial with all relevant evidence regarding the president’s conduct”.

He said he fears that senators “are headed towards a trial that is not intended to find the whole truth. For the sake of the country, this must change”.

Unlike what happened during the investigation of former president Bill Clinton, “Trump has blocked both the production of virtually all relevant documents and the testimony of witnesses who have firsthand knowledge of the facts”, Mr Jones said. “The evidence we do have may be sufficient to make a judgment, but it is clearly incomplete,” he added.

Mr Jones and other Democrats are seeking testimony from Mr Mulvaney and other key White House officials to help fill in the gaps.

Mr Romney is popular in conservative states where Mr Trump is not (Getty)

Mitt Romney

Mr Romney, a freshman senator and on-again, off-again Trump critic, has criticised Mr Trump for his comments urging Ukraine and China to investigate Democrat Joe Biden, but has not spoken directly about he thinks impeachment should proceed.

Mr Romney is overwhelmingly popular in a conservative state where Mr Trump is not beloved, a status that gives Mr Romney leverage to buck the president or at least speak out about rules and procedures of a Senate trial.

Cory Gardner

Mr Gardner, like Ms Collins is a vulnerable senator up for re-election in a state where Mr Trump is not popular. Mr Gardner has criticised the house impeachment effort as overly partisan and fretted that it will sharply divide the country.

While Mr Trump is under water in Colorado, a GOP strategist says Mr Gardner and other Republicans could benefit from an energised GOP base if the aenate, as expected, acquits Mr Trump of the two articles of impeachment approved by the House. An acquittal “may have a substantial impact on other races in Colorado, up to and including Sen. Cory Gardner’s re-election”, Ryan Lynch told Colorado Public Radio.

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Martha McSally

Ms McSally, who was appointed to her seat after losing a senate bid in 2018, is another vulnerable Republican seeking election this fall. She calls impeachment a serious matter and said she hopes her constituents would want her to examine the facts without partisanship.

The American people “want us to take a serious look at this and not have it be just partisan bickering going on”, she told The Arizona Republic.

Lamar Alexander

A three-term senator and former governor, Mr Alexander is retiring next year. A moderate who’s respected by both parties as an old-school defender of senate prerogatives, Mr Alexander has called Mr Trump’s conduct “inappropriate”, but says he views impeachment as a “mistake”.

An election, which “is just around the corner, is the right way to decide who should be president,” Mr Alexander said last fall. “Impeachment has never removed a president. It will only divide the country further.”