Washington: US House Speaker Nancy Pelosi is amplifying her unfavourable comparison of President Donald Trump to fellow Republican Richard Nixon, saying that the disgraced former president at least cared enough about the country to leave office before his impeachment.

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.