WASHINGTON (REUTERS) - Democratic prosecutors and Republican defenders in President Donald Trump's impeachment trial traded barbs in a second day of questioning by US senators on Thursday (Jan 30), with Democrats stressing the importance of calling witnesses such as former national security adviser John Bolton.

Without witnesses, Republicans, who control the Senate, say the trial could end as early as Friday with Trump's acquittal, which would leave him in office and allow him to claim vindication just as the Democratic Party holds its first nominating contest for the Nov 3 election in Iowa on Monday.

Trump will hold a rally in the state on Thursday night.

Democrats accuse the Republican president of abusing his power by using congressionally approved military aid as leverage to get a foreign power to smear former vice=president Joe Biden, a leading contender for the Democratic nomination.

The Democratic-led House of Representatives approved the two articles of impeachment in December. Lawyers for Trump and the House Democrats managing the prosecution answered questions from lawmakers, read aloud by US Chief Justice John Roberts.

On Friday, each side was expected to present what amount to closing arguments before the Senate moves to the question of whether to call witnesses. Democrats are unlikely to muster the two-thirds majority needed to remove Trump from office no matter what happens, but allowing witnesses could inflict political damage on the president as he seeks re-election.

Democrats believe witnesses are essential to shed more light on Trump's attempt to persuade Ukraine President Volodmyr Zelenskiy to investigate Biden and his son Hunter, who sat on the board of Ukrainian energy company Burisma.

Republican Senators David Perdue, Joni Ernst and John Barrasso asked Trump's lawyers to summarise the House probe "and how the president was denied due process in each stage ...do these due process violations make this impeachment the fruit of the poisonous tree?"

Republican Senator Ted Cruz asserted in one question that one of the House managers, Representative Val Demings, had refused to answer a question about Biden on Wednesday, then asked another question about the Bidens.

Demings countered she did not know the answer to his latest question, but swiftly brought it back to the question of witnesses.

"Maybe we should call Ambassador Bolton. If we are serious about the truth, maybe we should call him, because we have a good idea of what he might say," she said.

Possible testimony from Bolton is of particular interest 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 US$391 million (S$530 million) in US military aid for Ukraine until it investigated the Bidens.

WITNESSES

Republican Senator Josh Hawley said Republicans were becoming confident they had enough votes to defeat a motion to call witnesses and that the rules for the trial called for a "pretty quick transition" to a vote on acquittal afterward.

"The general sense in the conference is that it is more likely than not," he told reporters as he headed into the Senate chamber for the resumption of the proceedings.

Senate Democratic leader Chuck Schumer said he was still optimistic that witnesses could be called.

"I believe Senate Republicans and the president's team are worried about the vote," Schumer told a news conference on Thursday morning, before the proceedings restarted.

He also indicated that Democrats had a strategy to ensure the trial did not end on Friday, but would not provide details.

Democrats need to persuade at least four Republican senators to vote with them to assure a majority vote in the 100-seat chamber, an effort Schumer has called an uphill fight.

At least four Republicans - Lisa Murkowski of Alaska, Mitt Romney of Utah, Susan Collins of Maine and Lamar Alexander of Tennessee - are seen as potentially on the fence on the issue.

PRESIDENTIAL POWER

Democrats excoriated an expansive defence of presidential power offered by Trump lawyer Alan Dershowitz during Wednesday's question-and-answer session.

Dershowitz had said: "If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in an impeachment."

Representative Adam Schiff, the lead House prosecutor, told reporters as he headed to the Senate floor on Thursday: "Yesterday the president's defenders argued that the president of the United States could do whatever he wanted to secure his re-election - no matter how corrupt - if he believed his re-election was in the national interest... That is the most absurdly dangerous argument that could have been made."

Dershowitz tweeted on Thursday that the media "distorted" his remarks.

On Thursday, Roberts declined to read a question from Senator Rand Paul that may have named the anonymous whistleblower whose complaint started the Ukraine probe.

Lawyers for the whistleblower, a US intelligence official, say exposing his identity would violate federal law.

Paul said he was not given an explanation for the decision.

If the vote on whether to allow witnesses is 50-50, Roberts could step in to break the tie. But there is so little precedent for impeachment trials - this is only the third of a president in US history - that Senate aides said there was no way to know exactly what would occur.