WASHINGTON — Democrats warned Thursday that acquitting President Donald Trump of abuse-of-power charges in his impeachment trial would amount to the "normalisation of lawlessness."

In what could be their last bid to make the case for removing the US leader from office, the House impeachment prosecutors expressed outrage over Trump lawyer Alan Dershowitz's assertion that a president can do almost anything he wants that he claims is in the public interest.

Adam Schiff, the leader of the House impeachment managers, said the claim turned US law on its head and 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 told the Senate trial.

"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 spoke during the second day of questions submitted to the prosecution and defence from the 100 senators sitting as the jury.

Trump, only the third president in US history to be impeached and stand trial, is fighting charges of abuse of power and obstruction of Congress related to his scheme to pressure Ukraine last year to open an investigation into possible 2020 Democratic presidential candidate Joe Biden.

Trump is alleged to have frozen $391 million in defense aid to force Kiev to announce probes into Biden and the Democrats. Democrats say this is an illicit invitation of a foreign government to interfere in a US election.

Trump has repeatedly denied wrongdoing, calling the entire process a politically driven "witch hunt."

Speaking in Michigan on Thursday, Trump labelled the impeachment trial "frankly, a disgrace to our country."

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.  AFP