US Senate poised for crucial vote on witnesses at Trump trial

US senators wrapped up two days of exhaustive questioning of House prosecutors and White House lawyers at President Donald Trump's impeachment trial, setting the stage for a showdown vote on Friday on Democratic demands for new witnesses.

Chief Democratic prosecutor Adam Schiff, in a last-ditch bid to win over Republican senators wary of a drawn-out process, proposed a one-week time frame to secure new witness testimony.

"Let's have a reasonable accommodation here," the California lawmaker told the 100 senators sitting as jurors at the historic trial.

"We'll take one week, and you'll continue with the business of the Senate." Trump's defense team rejected the Democratic calls for more witnesses, saying the move would be challenged by the White House, sending the matter to the courts and paralysing the Senate for months.

"They said for weeks it was an overwhelming case," said White House deputy counsel Patrick Philbin.

"If that's their position, why do they need more witnesses?" Democrats are particularly eager to hear from former national security advisor John Bolton, who reportedly claims in an upcoming book that Trump personally told him that military assistance to Ukraine was tied to Kiev investigating his Democratic rival Joe Biden.

The charge is at the heart of the December 18 impeachment of the 45th US president for abuse of power and obstruction of Congress by the Democratic-majority House of Representatives.

Republicans hold a 53 to 47 seat edge in the Senate, and four Republicans will have to side with the Democrats for new witnesses to be introduced into the proceedings.

In the event of a 50-50 tie, US Supreme Court Chief Justice John Roberts, who is presiding over the trial, could be called upon to vote to end the deadlock. If he declines to vote, the motion calling for witnesses would fail.

A vote on witnesses will be held on Friday following four hours of debate, and it is unclear whether Democrats have mustered the necessary Republican support.

Utah Senator Mitt Romney said this week he would "very likely" vote for more witness testimony but only fellow Republican Susan Collins of Maine has also voiced support for the move.

If the demand for new witnesses is rejected, Republican Senate Majority Leader Mitch McConnell could conceivably call for a vote to convict or acquit the president as soon as Friday.

A two-thirds majority -- or 67 senators -- is needed to remove a president from office, and the solid Republican majority in the Senate means Trump is virtually certain to be acquitted.

"A final judgement of acquittal would be the best thing for our country," White House counsel Pat Cipollone said. "We're dealing with a purely partisan impeachment." Democrats were using impeachment "as a political weapon," he added, and voters -- not the Senate -- should decide Trump's fate in the November presidential election.

"We're talking about removing a president of the United States from the ballot in an election that's occurring in months," he said.

During the occasionally testy eight-hour question-and-answer period on Thursday, the House impeachment managers warned that acquitting Trump would amount to the "normalization of lawlessness."

They also expressed outrage over an assertion by one of Trump's lawyers, Harvard professor emeritus Alan Dershowitz, that a president can do almost anything he wants if he believes his re-election was "in the public interest."

"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 impeachment," Dershowitz said.

Schiff said the claim echoed the Watergate case of disgraced president Richard Nixon 45 years ago.

"What we have seen in the last couple of days is a descent into constitutional madness," Schiff said.

"Almost half a century ago, we had a president who said, 'Well, when the president does it that means it is not illegal.'" "Have we learned nothing in the last half century?" Schiff asked. "It seems like we're back to where we were.

"That is the normalization of lawlessness," he said. "I would hope that every American would recognize that it's wrong to seek foreign help in an American election."

Trump's defense team has argued alternatively that the impeachment is politically driven, that Trump did not tie Ukraine aid to the investigations he sought, that he had reason to seek a probe into his political rival and that he had the legal right to do so as president.

The president has repeatedly denied wrongdoing, calling the entire process a politically driven "witch hunt" and a "disgrace."

Speaking at a campaign rally on Thursday in Des Moines, Iowa, Trump said, "We're having the best years in the history of our country and I just got impeached."

Democrats are to hold their first vote in Iowa on Monday in the process of selecting a candidate to face Trump in November's presidential election.