WASHINGTON (REUTERS) - United States House Speaker Nancy Pelosi is amplifying her unfavourable comparison of President Donald Trump to fellow Republican Richard Nixon, saying that the disgraced president at least cared enough about the country to leave office before his impeachment.

The top Democrat in Congress told reporters last week that Mr Trump's pressure on Ukraine to investigate one of his potential opponents in the 2020 election "makes what Nixon did look almost small".

In a CBS interview broadcast on Sunday (Nov 17), she alluded to Mr Nixon's resignation after the Watergate scandal involving a break-in at Democratic Party headquarters and the subsequent cover-up.

"I mean, what the President did was so much worse than even what Richard Nixon did, that at some point Richard Nixon cared about the country enough to recognise that this could not continue," Ms Pelosi said on CBS' Face The Nation.

Mr Nixon, whose name has become synonymous with scandal and ignominy for many Americans, resigned in 1974 after the House Judiciary Committee approved articles of impeachment against him but before the full House voted on the issue, and he was not impeached.

He is the only US president who has resigned from office.

Ms Pelosi for months resisted calls from her more liberal Democratic lawmakers to initiate impeachment proceedings, but said Mr Trump's call with Ukrainian President Volodymyr Zelensky compelled her to open the inquiry against him.

Since launching the proceedings on Sept 24, Ms Pelosi has not been in the room as the House Intelligence Committee held public hearings on Mr Trump's impeachment. However, her voice is loud and clear on the outside, where she drives messaging in a nuanced but sharp manner.

Her Nixon comparison came amid the trial of long-time Trump ally Roger Stone, a self-proclaimed "dirty trickster" who worked for Mr Nixon's re-election campaign and has the former president's face tattooed on his back. Stone was convicted last Friday of lying to Congress, obstruction and witness tampering during the investigation into Russian interference in the 2016 US election.

Mr Trump and his supporters have attacked the impeachment probe as politically motivated. Mr Trump says his call with Mr Zelensky was "perfect" while Republican lawmakers criticise the impeachment process as unfair.

"Do you have any evidence at all that the President did anything criminal or illegal? And the answer is no," Republican US Representative Chris Stewart said on ABC's This Week.

The President has "every opportunity to present his case", Ms Pelosi told CBS, including coming before the intelligence panel.

"If the President has information that demonstrates his innocence in all of this, which we haven't seen," she said. "If he has information that is exculpatory - that means ex, taking away, culpable, blame - then we look forward to seeing it."

Mr Trump unleashed a daylong flurry of three dozen tweets and retweets on Sunday, most of them critical of the impeachment. "The Crazed, Do Nothing Democrats are turning Impeachment into a routine partisan weapon," he wrote.

Ms Pelosi accused Mr Trump of bribery last week in having his aides dangle a White House meeting, then US$400 million (S$544 million) in suspended US security assistance, if Mr Zelensky announced an investigation into a Democratic 2020 political rival, former vice-president Joe Biden. Bribery is one of three articles of impeachment in the US Constitution.

In the CBS interview, taped last Friday, the House Speaker called Mr Trump an "imposter" whose insecurity drove his real-time Twitter attack on former US ambassador to Ukraine Marie Yovanovitch as she testified in the impeachment inquiry.

"Everywhere Marie Yovanovitch went turned bad," Mr Trump said last Friday as she testified, an extraordinary moment that Democrats said amounted to witness intimidation.

Republicans at the hearing expressed support for Ms Yovanovitch's public service and some later openly criticised Mr Trump's actions.

"I find the President's tweet unfortunate," Representative Mike Turner, a Republican on the intelligence panel, said on CNN's State Of the Union.

"He made a mistake," Ms Pelosi told CBS. "He knows her strength. And he was trying to undermine it."