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

Adam Schiff, a Democrat from California, is the chief House prosecutor at the Senate impeachment trial of President Donald Trump Adam Schiff, a Democrat from California, is the chief House prosecutor at the Senate impeachment trial of President Donald Trump Photo: GETTY IMAGES / Drew Angerer

"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?

Harvard professor Alan Dershowitz, a member of President Donald Trump's legal team Harvard professor Alan Dershowitz, a member of President Donald Trump's legal team Photo: GETTY IMAGES / MARIO TAMA

Schiff spoke during the second day of questions submitted to the prosecution and defense 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.

Protesters outside the US Capitol during the Senate impeachment trial of US President Donald Trump Protesters outside the US Capitol during the Senate impeachment trial of US President Donald Trump Photo: AFP / Andrew CABALLERO-REYNOLDS

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.

On Wednesday Dershowitz, a famous criminal defense lawyer and former Harvard University professor, declared that Trump could make those demands if he thought his reelection was "in the public interest."

US President Donald Trump was impeached in December 2019 by the Democratic-controlled House of Representatives on charges of abuse of power and obstruction of Congress US President Donald Trump was impeached in December 2019 by the Democratic-controlled House of Representatives on charges of abuse of power and obstruction of Congress Photo: AFP / SAUL LOEB

"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," he said.

Democrats and much of the US legal community were stunned by the claim.

"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.

Democrats are seeking the testimony of former national security advisor John Bolton at President Donald Trump's Senate impeachment trial Democrats are seeking the testimony of former national security advisor John Bolton at President Donald Trump's Senate impeachment trial Photo: AFP / Brendan Smialowski

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

"It seems like we're back to where we were. The president says it, it's not illegal," said Schiff.

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

On Thursday, Trump's defense team avoided the issue and repeatedly argued that it was improper to be seeking a popular president's removal by trial in an election year.

White House Counsel Pat Cipollone said Democrats were using impeachment "as a political weapon" and that voters, and not the Senate, should decide Trump's fate in the November election.

"Let's leave it to the people of the United States.... That's who should decide who the president of this country should be. It will be a few months from now, and they should decide," he said.

Eric Herschmann, another of Trump's lawyer, appeared to argue that Trump's popularity and policy success invalidated any attempt to convict him at trial.

"The president's approval rating while we are sitting here in the middle of these impeachment proceedings has hit an all-time high," Herschmann said.

"We, the American people, are happier," he said. "We trust the American people to decide who should be our president."

Democrats admitted they were fighting an uphill battle to convince Republicans to go against Trump.

After Thursday's question session, they will have a few hours Friday to push for Senate subpoenas of direct witnesses to Trump's alleged wrongdoing.

First on their list is former White House national security advisor John Bolton, who reportedly claims in a yet-to-be-published book that Trump told him personally that military aid to Ukraine was tied to Kiev's investigating Biden.

But Democrats need to get a majority of the Senate to support a witness call, and Republicans control 53 seats, while the Democrats have 47.

Both sides were pressuring a handful of Republican senators on the issue, but by Thursday it was not clear that Democrats had secured the four crossovers they need in order to call witnesses.

If Democrats fail on the witnesses, Republicans are expected to bring the trial to final votes on the two impeachment charges.

With conviction and removal of the president requiring a two-thirds majority, Trump's acquittal is virtually assured.