US President Donald Trump (C) attends a signing ceremony of the United States-Mexico-Canada Agreement (USMCA) at the White House in Washington D.C., the United States, on Jan. 29, 2020. U.S. President Donald Trump on Wednesday signed the revised United States-Mexico-Canada Agreement (USMCA) in an outdoor ceremony at the South Lawn of the White House. (Xinhua/Liu Jie)

 

The US Senate is expected to wrap up the initial phase of President Donald Trump's impeachment trial before turning on Friday to the explosive question of whether to call witnesses such as former national security adviser John Bolton.

Republicans who control the Senate said there was a chance the trial could end on Friday with Trump's acquittal, leaving him in office and rendering moot the articles of impeachment Democrats approved in the House of Representatives in December.

An acquittal would allow the Republican president to claim vindication and put the threat to his presidency behind him just as Americans begin focusing on the Democratic race to choose his challenger in the November 3 election.

Iowa holds the first election contest of the year on Monday. Trump will be there on Thursday night for a rally to promote his record and skewer his opponents.

Lawyers for Trump and the House Democrats who are managing the impeachment prosecution will spend a second day on Thursday answering questions about the case written down by lawmakers and read aloud by black-robed US Chief Justice John Roberts.

Then, probably on Friday, each side will present what amount to closing arguments, before the senators move to the central question of whether to call witnesses.

Democrats are demanding witnesses in order to shed more light on Trump's attempt to persuade Ukrainian President Volodmyr Zelenskiy to investigate Democratic rival Joe Biden.

To force the issue, they need to persuade at least four Republican senators to vote with them to assure a majority vote in the 100-seat chamber, an effort the top Democrat in the Senate has called an uphill fight.

Even if witnesses are called, Democrats hold out scant hope of ultimately mustering the two-thirds majority needed to remove Trump from office.

Bolton this week emerged as a potential witness after a report - which he has not ­denied - that he planned to say in an upcoming book that Trump told him he wanted to freeze $391 million in US military aid for Ukraine until it investigated Biden and the former vice president's son, Hunter.

Democratic Representative Jerrold Nadler of New York argued that Trump wanted to "rig an election" by inviting foreign interference in the 2020 election.

"The facts are the facts," said Nadler. 

"The president is a danger to the United States... and he must be brought to heel."