A claim by one of Donald Trump's lawyers that the head of state has virtually unchallengeable power inflamed his impeachment trial as Democratic prosecutors prepared on Thursday for a last bid to breach Republican loyalty to the president.

Famed criminal lawyer Alan Dershowitz on Wednesday advanced an argument that Trump's actions did not constitute an impeachable abuse of power because the president believed his re-election was in the public interest.

Dershowitz, a former Harvard University law professor, was addressing the central charge against Trump: that he illicitly pressured Ukraine to open investigations against rival Democrats and particularly his possible 2020 re-election challenger Joe Biden.

"Every public official I know believes that his election is in the public interest," Dershowitz told the 100 senators sitting in judgment at the historic trial - in which Trump is hoping the Senate's Republican majority will ensure a speedy acquittal.

"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."

Impeached on December 18 by the Democratic-held House of Representatives, Trump is also accused of freezing $391 million in military aid to Ukraine in order to force President Volodymyr Zelensky to announce the investigations.

Dershowitz's argument stunned House impeachment managers prosecuting the case and the broad US legal community.

Sounds like North Korea

As summed up by Adam Schiff, the head of the Democratic prosecution team, it amounted to saying that: "If it's good for him (Trump), it's good for the state, because he is the state."

"I can't think of anything more dangerous to our democracy," said Neal Katyal, a former top Justice Department official and a law professor at Georgetown University.

"Every president can say heck, I'm doing this in the nation's interest."

"It doesn't pass a common sense test.... That sounds like something coming out of North Korea, not Pennsylvania Avenue," Democratic Senator Tammy Duckworth told MSNBC.

In a tweet early Thursday Dershowitz accused media of "willfully" distorting his arguments during the trial questioning, claiming he "said nothing like" what was being alleged.

Majority rule

Dershowitz's defence of Trump came on the first of two marathon days of questioning of both the prosecutors and defence by senators in the trial.

Thursday's questioning could be House impeachment managers' last chance to make the case that Trump should be convicted and removed.

Republicans, who have a 53-47 Senate majority, are hoping to bring the trial to a close on Friday with a vote to acquit Trump.

Democrats are hoping to prolong it by convincing Republicans to agree to call witnesses with direct knowledge of Trump's Ukraine dealings, in particular his former national security advisor John Bolton.

According to reports, Bolton's forthcoming book said Trump personally told him in August that aid to Ukraine was frozen to pressure Kiev to investigate Biden.

The National Security Council has said a preliminary review of Bolton's book found that it contained "significant amounts of classified information" and cannot be published in its current form.

Executive privilege

In a vote on witnesses expected to come Friday, Democrats need four Republicans to break with their party to force a call for witnesses.

While some Republican senators such as Mitt Romney have said they favor calling Bolton, it was not clear Thursday that enough of them held that view to force the issue.

But several were reportedly under heavy pressure from the Republican leadership to stand in line with their party and bring the trial to a quick conclusion.

Republicans meanwhile levelled a double threat to Democrats if there is a decision to call witnesses: that the White House would claim executive privilege to block Bolton from appearing, and that they would also subpoena Biden and his son Hunter to testify.

"Of course, if witnesses are called by the House managers through that motion, the president's counsel would have the opportunity to call witnesses as well, which we would," said Trump lawyer Jay Sekulow.