President Trump's defense team completed its arguments with an appeal for a quick acquittal in the Senate impeachment trial as Republican U.S. senators remained uncertain over the key question of whether to call witnesses.

'An unsourced allegation'

Saying "it is time for this to end," Trump's lawyers brushed off former national security adviser John Bolton's explosive allegations about Trump's conduct and accused Democrats of trying to interfere with Trump's November re-election bid.

Trump's attorney Jay Sekulow sought to dismiss the published revelations from a draft of Bolton's forthcoming book that contradicts key defense arguments from the White House about Trump's dealings with Ukraine and calling it "inadmissible."

"You cannot impeach a president on an unsourced allegation," Sekulow told the Senate.

Bolton's allegations go to the heart of impeachment charges against Trump. Democrats have said Trump abused his power by using the security aid – approved by Congress to help Ukraine battle Russia-backed separatists – as leverage to get a foreign power to smear a political rival.

Sekulow underscored what fellow Trump legal team member Alan Dershowitz told senators late on Monday – that even if what Bolton said was true, it would not represent impeachable conduct.

Bolton left his White House post last September. Trump has said he fired Bolton and Bolton said he quit after policy disagreements.

Trump has denied telling Bolton he sought to use the Ukraine aid as leverage to get Kiev to investigate the Bidens. He has denied any quid pro quo – a Latin term meaning a favor for a favor – in his dealings with Ukraine.

Other members of Trump's legal team, including attorney Alan Dershowitz, said that even if the information in the book was true, it would still not rise to the level of an impeachable offense.

The Democratic-led House on December 18 impeached Trump on charges of abuse of power and obstruction of Congress arising from his request that Ukraine would investigate Biden, the former vice president.

'Serious and lasting damage'

"Overturning past elections and massively interfering with the upcoming one would cause serious and lasting damage to the people of the United States and to our great country. The Senate cannot allow this to happen," White House Counsel Pat Cipollone told the Senate.

"It is time for this to end, here and now. So we urge the Senate to reject these articles of impeachment."

When they reconvene on Wednesday, senators will begin two days of questions to the lawyers representing Trump and to the seven House of Representatives Democrats who have served as prosecutors. That would leave summations and a vote on witnesses for Friday.

Adam Schiff, who served as the lead Democratic prosecutor in arguing the case against Trump last week, said witnesses would be needed for the trial to be considered fair.

"A fair trial involves witnesses and it involves documents," he told reporters.

Bolton's manuscript directly contradicts Trump's account of events. He wrote the president told him he wanted to freeze 391 million U.S. dollars in security aid to Ukraine until Kiev pursued investigations into Democrats, including Biden and his son Hunter Biden, the New York Times reported.

Read more:

Trump lawyers call for immediate acquittal in legal, political defense

White House lawyers at Senate trial say Trump's fate should be decided by voters

Trump defense lawyers attack Bidens at impeachment trial

Uncertainty on witnesses

Republican senators met behind closed doors to discuss calling witnesses including Bolton, but emerged with no certainty. Four Republicans would need to vote for witnesses, along with all 47 Democrats and independents.

Republican Senator John Barrasso said the consensus was "we've heard enough and it's time to go to a final judgment vote." But other Republicans said the vote count was unclear and no decision would be made until Friday.

Republican Senator Kevin Cramer, a conservative defender of Trump who opposes witnesses, said Republicans were "mostly united." "I'm pretty sure it's not unanimous. But I don't know what the numbers are."

Some Republican senators who oppose calling witnesses proposed that Bolton's manuscript be made available for senators to review on a classified basis, an idea rejected by top Senate Democrat Chuck Schumer.

"What an absurd proposal. It's a book," Schumer told reporters about the proposal floated by Republican Senators Lindsey Graham and James Lankford, saying there was no need to place the manuscript for review in a classified setting "unless you want to hide something."

Lankford urged Bolton to speak publicly outside of the trial.

Schumer criticized Trump's legal team for stating during its arguments to the Senate that there was no eyewitness testimony detailing abuse of power by Trump. "When we know that John Bolton has eyewitness testimony and is willing to testify."

Schumer made a fresh appeal for four Republican senators – the number needed for a majority – to join Democrats in voting to call witnesses. He also indicated Democrats would reject any effort at a so-called witness swap with Republicans.

"The Republicans can call who they want. They have the ability. They have the majority," Schumer said.

A partisan exercise

With Trump's legal team concluding its defense, the case now moves toward written questions, with senators getting 16 hours to pose questions. By the end of the week, they're expected to hold a vote on whether to subpoena any witnesses.

Sekulow sought to portray Trump as the victim of scheming by Washington insiders dating back to his 2016 candidacy. He listed grievances that Trump raised about prior investigations including the special counsel probe that documented Russian interference in the 2016 election to boost his candidacy and his campaign's numerous contacts with Moscow.

The impeachment drive against Trump, Sekulow argued, was a partisan exercise motivated by Democratic opposition to Trump's policies, not genuine impeachable offenses.

"But to have a removal of a duly elected president based on a policy disagreement?" Sekulow asked. "That is not what the framers (of the Constitution) intended. And if you lower the bar that way – danger, danger, danger. Because the next president or the one after that, he or she would be held to that same standard? I hope not. I pray not."

(With input from Reuters)