Washington, DC - The Senate impeachment trial of President Donald Trump enters a critical phase on Friday as House Democrats make their best argument why former National Security Adviser John Bolton should testify.

While Republicans appear unified so far in support of their president, Democrats hope that by detailing Trump's obstruction of their Ukraine inquiry they can convince a small faction of Republicans to ask for Bolton's testimony.

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If Bolton appears, it could be explosive, legal scholars and analysts say.

"If Mr Bolton were to testify, he would blow the roof off the US Capitol," said Gene Rossi, former federal prosecutor now in private practice at Carlton Fields law firm in Washington, DC.

"Bolton would describe in intimate detail the minutiae of interactions between the president and his conspirators to execute the arms-for-dirt scheme he called a 'drug deal,'" Rossi told Al Jazeera.

The House of Representatives impeached Trump in December for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to participate in the impeachment probe. Democrats accuse the president of abusing his power in office by leveraging a White House meeting and withheld military assistance to get Ukraine to announce investigations in Trump's political rival Joe Biden.

Democrats on Friday are expected to focus on the obstruction charge, while making the case for witnesses, including Bolton and acting White House Chief of Staff Mick Mulvaney.

With Republicans in control of the Senate by a 53-47 margin, Democrats are not expected to garner the two-thirds vote necessary to convict Trump on articles of impeachment.

But Democrats believe Bolton's testimony could give enough Republicans reason to create at least a majority vote to convict, which would be a huge political victory.

Bolton is among several witnesses Trump prevented from testifying during the House impeachment inquiry, including acting Chief of Staff Mick Mulvaney and other White House officials.

'Hand grenade'

As Trump's national security adviser, Bolton participated in direct conversations with the president, Rudy Giuliani, Trump's personal lawyer, and other key players in the impeachment probe. Bolton was forced out of the White House after several disagreements with Trump.

"The reason that Bolton is an important witness is that the Senate majority has pre-judged the case and indicated that they will not vote to remove President Trump," said Barb McQuade, a former federal prosecutor and law professor at the University of Michigan.

"If he were to testify about new details regarding what he referred to as a 'drug deal' and a 'hand grenade," it is possible that some senators would change their minds," McQuade told al Jazeera.'

Republican Senators Mitt Romney, Susan Collins and Lisa Murkowski have signalled they may be willing to join the Democrats on a motion to call witnesses later in the trial. A fourth Republican would be needed to get to the 51 votes needed. Senator Lamar Alexander could provide that swing vote.

Trump speaks as National Security Adviser John Bolton listens during a meeting [Oliver Contreras/EPA]

Bolton has said he would testify if subpoenaed. Trump has said he would seek to assert executive privilege over Bolton's testimony, potentially complicating and extending the Senate trial.

Much of the case revolves around the actions of Trump's personal lawyer Giuliani, who Bolton called "a hand grenade who's going to blow us all up", according to testimony.

Giuliani claims to have information on former Vice President Biden and his son Hunter Biden, a former board member of Ukrainian gas company Burisma. He travelled to Kyiv in December and tweeted on Thursday that he would begin to disclose details.

Democrats argued in the Senate on Thursday that Giuliani's allegations about Biden in Ukraine are "completely debunked" conspiracy theories. In fact, there is no evidence the Bidens did anything wrong but Republicans are continuing to push the issue.

"When I say I like Joe Biden, I'm not kidding," Senator Lindsey Graham, a Trump ally, told reporters on Thursday.

"But here's the problem. If you're the vice president of the United States and the Ukraine is in your portfolio, you're going to have to answer questions about how your son made three million dollars from the most corrupt company in the Ukraine, you didn't know about it," Graham said.

In a rare moment of levity during the Senate trial on Thursday, House manager Sylvia Garcia drew involuntary laughter from Republicans when she played a video clip of former a White House aide joking about Giuliani.

Tom Bossert, a former Homeland Security adviser to Trump, said in a television interview in September that there "are three ways to impeach one's self. And the third way was to hire Rudy Giuliani."

That drew a smirk from the usually stone-faced Republican Majority Leader Mitch McConnell as a row of Republicans behind him laughed out loud.

Restless days

Some Republican senators complained of boredom during the lengthy presentations by House Democrats on Thursday.

Senator Richard Burr handed out fidget spinners to a few colleagues and he was seen openly spinning a light blue spinner for half an hour as House manager Jerry Nadler, a Democrat, expounded on the constitutional legal basis for Trump's impeachment.

After Friday, the president's defence team will have 24 hours over three days to mount a rebuttal of the Democrat's case.

Trump's lawyers are not expected to directly address the House's evidence in the case, but likely will criticise the House impeachment process as partisan and unfair.

"I'm hoping our side will simplify it more and not go into as laborious and complicated arguments," said Republican Senator Mike Braun.

After having been treated unbelievably unfairly in the House, and then having to endure hour after hour of lies, fraud & deception by Shifty Schiff, Cryin’ Chuck Schumer & their crew, looks like my lawyers will be forced to start on Saturday, which is called Death Valley in T.V. — Donald J. Trump (@realDonaldTrump) January 24, 2020

Trump again on Friday went on a tweet storm, railing against the Democrats and calling the impeachment a "hoax".

US public opinion on Trump's impeachment is sharply divided between Democrats and Republicans with a slight tilt in favour of removing the president from office.

A new Emerson College national poll showed 51 percent of respondents favour Trump's conviction and removal from office while 49 percent oppose impeachment.

A strong majority of 59 percent of voters want the Senate to call witnesses in the trial, according to the survey conducted January 21-23 with a 4.1 percent margin of confidence.