The top Democrat in Congress told reporters last week that 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, she alluded to 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," Pelosi said on CBS' Face the Nation.

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.

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

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

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

The president has "every opportunity to present his case," 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."

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.

Pelosi accused Trump of bribery last week in having his aides dangle a White House meeting, then $US400 million in suspended US security assistance, if Zelenskiy 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.

Australian Associated Press