A top Democrat said Wednesday that President Donald Trump's July phone call pressing Ukraine's leader to investigate Trump's Democratic rival Joe Biden resembled a mafia boss's corrupt squeeze on a victim.

The notes of the call released by the White House "reflect a classic mafia-type shakedown of a foreign leader," said Adam Schiff, who leads one of the House committees involved in the impeachment investigation of Trump.

The July 25 call showed Trump urging Ukraine President Volodymyr Zelensky to investigate Biden and other matters, referring to a "favor" without committing to giving what Zelensky sought, military aid which Trump had frozen just days before.

"This is how a mafia boss talks: 'What have you done for us? We've done so much for you but there's not much reciprocity. I have a favor to ask you,'" Schiff said.

"And what is that favor? Of course, the favor is to investigate his political rival, to investigate the Bidens."

"Ukraine knew what it needed to do if it wanted to get military assistance."

"The notes of the call reflect a conversation far more damning than I or many others had imagined," said Schiff, whose House Intelligence Committee is investigating Trump's dealings with Ukraine.

Schiff said the call notes, released one day after Democrats opened an impeachment probe of the president, showed Trump seeking foreign help to support his 2020 campaign for reelection, which can be illegal.

That Trump "would abuse his power to seek the help of another nation in his campaign is the most fundamental betrayal of his oath of office," Schiff said.

"The president of the United States has betrayed his oath of office and sacrificed our national security in doing it."