Washington, DC - The United States Senate nears the end of the first phase of the impeachment trial President Donald Trump on Thursday, with a possible eight more hours of questions before senators address the crucial question of whether witnesses will be called.

Senators used half of 16 hours on Wednesday they have available over two days to ask written questions of the House managers prosecuting the case and the president's defence team.

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Both Republicans and Democrats are using the opportunity to pose questions to amplify or dispute key legal and factual issues. Trump was impeached for abuse of power and obstruction of Congress.

Several of the questions on Wednesday centred on former White House National Security Adviser John Bolton, who reportedly wrote in a forthcoming manuscript that Trump said he was withholding aid to Ukraine in exchange for political investigations of Biden. Bolton has said he is willing to testify to the Senate if subpoenaed.

Lead House Manager Adam Schiff called Bolton a "central witness" and told senators that White House efforts to prevent him from appearing in trial "screams at you" the president does not want his testimony to come out.

Republican leaders argue calling witnesses could lead to a lengthy trial with court battles, delaying what is ultimately expected to be an acquittal.

Trump listens as his then-national security adviser, John Bolton, speaks during a presidential memorandum [File: Leah Millis/Reuters]

The White House took a step on Wednesday to temporarily block publication of Bolton's book by advising his publisher the manuscript contains top-secret information. Bolton's lawyer pushed back, saying it does not.

The Senate is poised to vote as soon as Friday on the question of witnesses. In the hallways outside the chamber, Republicans suggested that there was "momentum" for the trial to end as soon as Friday with an acquittal and no witnesses. It is unclear, however, if Trump's party has the votes to block Bolton and other witnesses. Democrats need at least four Republicans to vote with them for witnesses.

"There's tremendous pressure from a vindictive, nasty president on every Republican senator, but I think (as) they sit there ... we've got a real shot to get witnesses and documents," Senate Democratic leader Chuck Schumer said.

Republican Senator Susan Collins submitted the first question on Wednesday, along with Senators Lisa Murkowski and Mitt Romney. All three have expressed interest in calling Bolton.

"How should the Senate consider more than one motive in its assessment of article one?" Collins asked, referring to the abuse of power article of impeachment.

This artist sketch depicts the Republican side of the Senate during defence arguments in the impeachment trial of President Donald Trump [Dana Verkouteren/AP Photo]

Trump lawyer Patrick Philbin responded that once it's established there is at least a portion of "legitimate public interest" behind the president's actions, the abuse of power allegations are null.

"Once you're into mixed-motive land, it's clear that their case fails," Philbin said.

Schiff, asked by Democrats to respond, argued that if Trump's corrupt political desires were "in any part a causal factor" in his actions, "that's enough to convict."

Senators asked dozens of other questions on Wednesday touching on the strength of the House's case, the whistle-blower's complaint first exposed the Ukraine affair, and former Havard professor Alan Dershowitz's constitutional argument Trump is not impeachable.

Regardless of witnesses, Trump is expected to ultimately be acquitted as Democrats are unlikely to muster the two-thirds majority vote in the 100-member Senate to convict and remove the president. Republicans hold a 53-47 majority.

But a witness, such as Bolton could potentially hurt Trump as he heads towards November's 2020 presidential election.

While Americans remain divided on the question of whether Trump should be convicted in the impeachment trial, recent polls show a growing majority supports calling witnesses.

A new poll released by The Economist/YouGov showed 44 percent of Americans think the Senate should remove Trump from office, while 43 percent say the Senate should not.

At the same time, 49 percent said the Senate should consider new evidence, while only 32 percent said the Senate should not with 19 percent undecided.

The survey of 1,500 adults was conducted January 26-28 with a 2.8 percent margin of confidence.