US Senators faced mounting pressure on Monday to summon former National Security Advisor John Bolton to testify at President Donald Trump's impeachment trial even as Trump's lawyers brushed past extraordinary new allegations from Bolton and focused instead on corruption in Ukraine and historical arguments for acquittal.

Bolton took centre stage on the seventh day of the trial, even though he was nowhere near the United States Capitol building.

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Inside the Senate chamber, Trump's defence team mounted more than seven hours of arguments in favour of the president, but outside, Republicans grappled with claims in a forthcoming book from Bolton that Trump had wanted to withhold military aid from Ukraine until it committed to helping with investigations into Democratic rival Joe Biden.

That assertion could undercut a key defence argument - that Trump never tied the suspension of security aid to political investigations.

The revelation clouded White House hopes for a swift end to the impeachment trial, heightening Democratic demands for witnesses and possibly pushing more Republican legislators to agree. It also distracted from hours of arguments from Trump's lawyers, who declared anew that no witness has testified to direct knowledge that Trump's delivery of aid was contingent on investigations into Democrats. Bolton appeared poised to say exactly that if called on by the Senate to appear.

This still image taken from a US Senate webcast shows Counsel to the President Jay Sekulow speaking in the Senate Chamber at the US Capitol during the impeachment trial [US Senate TV/AFP]

"We deal with transcript evidence, we deal with publicly available information," lawyer Jay Sekulow said. "We do not deal with speculation."

Trump is charged with abusing his presidential powers by asking Ukraine's leader to help investigate Biden at the same Trump was ordering that millions of dollars in aid be withheld. A second charge accuses Trump of obstructing Congress in its probe.

'No basis' to remove Trump

Trump's legal team on Monday launched a wide-ranging attack on the entire impeachment process. They said there was no basis to remove him from office, defended his actions as appropriate and assailed Biden, who is campaigning for the Democratic nomination to oppose Trump in November.

The lawyers focused particular attention on Biden and his son, Hunter, who served on the board of a Ukraine gas company at the same time his father was vice president. They argued that Trump had legitimate reasons to be suspicious of the younger Biden's business dealings and concerned about corruption in Ukraine and that, in any event, he ultimately released the aid without Ukraine committing to investigations the president wanted.

Democrats say Trump did so only after a whistle-blower submitted a complaint about the situation.

Ken Starr, whose independent counsel investigation into President Bill Clinton resulted in his impeachment - Clinton was acquitted by the Senate - bemoaned what he said was an "age of impeachment".

Impeachment, he said, requires both an actual crime and a "genuine national consensus" that the president must go. Neither exists here, Starr said.

"It's filled with acrimony and it divides the country like nothing else," Starr said of impeachment. "Those of us who lived through the Clinton impeachment understand that in a deep and personal way."

Debate within Republican party

Even as defence lawyers laid out their case as planned, it was clear that Bolton's book had scrambled the debate over whether to seek witnesses. Bolton writes that Trump told him he wanted to withhold security aid from Ukraine until it helped him with investigations. Trump's legal team has insisted otherwise, and Trump tweeted on Monday that he never told Bolton such a thing.

Republican senators face a pivotal moment. The pressure is mounting for at least four to buck Republican leaders and form a bipartisan majority to force the issue. Republicans hold a 53-47 majority.

"John Bolton's relevance to our decision has become increasingly clear," Republican Senator Mitt Romney told reporters. Senator Susan Collins said she has always wanted "the opportunity for witnesses" and the report about Bolton's book "strengthens the case".

At a private Republican lunch, Romney made the case for calling Bolton, according to a person who requested anonymity because they were unauthorised to discuss the meeting.

Other Republicans, including Senator Pat Toomey of Pennsylvania, said if Trump's former national security adviser is called they would demand reciprocity to hear from at least one of their witnesses. Some Republicans want to call the Bidens.

Senate Majority Leader Mitch McConnell appeared unmoved by news of the Bolton book. His message at the lunch, said Republican Senator Mike Braun, was, "Take a deep breath and let's take one step at a time."

Once the president's team wraps its arguments no later than Tuesday, senators have 16 hours for questions to both sides.

By late in the week, they are expected to hold a vote on whether or not to hear from any witnesses.

'White House cover-up'

While Democrats say Bolton's revelations are reminiscent of the Watergate drip of new information, Republicans are counting on concerns subsiding by the time senators are asked to vote. They are being told that if there is an agreement to summon Bolton, the White House will resist, claiming executive privilege. That would launch a weeks-long court battle that could drag out the impeachment trial, a scenario some Republican senators would rather avoid.

Trump and his lawyers have argued repeatedly that Democrats are using impeachment to try to undo the results of the last presidential election and drive Trump from office.

Trump tweeted for viewers to tune in to the "hoax", advertising the trial's start time.

Senate Minority Leader Chuck Schumer joined by Senator Ben Cardin speaks during a news conference about the impeachment trial of President Donald Trump on charges of abuse of power and obstruction of Congress [J Scott Applewhite/AP Photo]

Some in the White House had hoped the legal team would steer away from the Bidens but acknowledged the Bolton revelations contributed to the decision to stay focused on the family. They worry about squandering what goodwill they have earned with the Senate, where Biden served for decades.

Democrats meanwhile say Trump's refusal to allow administration officials to testify only reinforces that the White House is hiding evidence. The White House has had Bolton's manuscript for about a month, according to a letter from Bolton's lawyer.

Senate Democratic leader Chuck Schumer said: "We're all staring a White House cover-up in the face."

Representative Adam Schiff, who leads the House prosecution team, called Bolton's account a test for the senators sitting as jurors.

"I don't know how you can explain that you wanted a search for the truth in this trial and say you don't want to hear from a witness who had a direct conversation about the central allegation in the articles of impeachment," Schiff said on CNN.

Bolton's account was first reported by The New York Times newspaper and was confirmed to The Associated Press news agency by a person familiar with the manuscript on the condition of anonymity. The Room Where It Happened; A White House Memoir is to be released March 17.

Trump denied Bolton's claims in tweets Monday.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump said. "If John Bolton said this, it was only to sell a book."

Bolton looks on as he meets with journalists during a visit to London [File: Peter Nicholls/Reuters]

Joe Biden, campaigning in Iowa, said he sees no reason for testimony by him or his son.

"I have nothing to defend. This is all a game, even if they bring me up," he told reporters. "What is there to defend? This is all - the reason he's being impeached is because he tried to get a government to smear me and they wouldn't. Come on."

Trump said people could look at "transcripts" of his call with Ukrainian President Volodymyr Zelenskyy to see there was no pressure for investigations to get the aid. According to a summary of the call, Trump asked Zelenskyy to "do us a favour" with the investigations as he was withholding nearly $400m in military aid to the US ally at war with Russia.

Trump falsely claimed on Monday that the Democrat-controlled House "never even asked John Bolton to testify". Democrats did ask Bolton to testify, but he did not show up for his deposition. They later declined to subpoena Bolton, as they had others, because he threatened to sue, which could have led to a prolonged court battle.

Eventual acquittal is likely in a Senate where a two-thirds majority vote would be needed for conviction

Democrats argued their side of the impeachment case for three days last week, warning that Trump would \persist in abusing his power and endangering American democracy unless Congress intervenes to remove him before the 2020 election.