Democrats claim that on July 25, in a call with Mr Zelenskiy, Mr Trump sought to leverage $391 million in US security aid to Ukraine to secure the Biden investigation. Mr Trump denies doing so.
Three scholars, called by Democrats on the judiciary committee, declared themselves “unanimous” that Mr Trump had abused his power and should be impeached.
In doing so they introduced references to historical figures including Louis XIV, Charles II, Sir Thomas More, and Alexander Hamilton.
Impeachment had been intended by the Constitution’s framers as a protection against monarchy, dictatorship, and corruption of elections, they said.
Michael Gerhardt, professor of constitutional law at the University of North Carolina, said Mr Trump’s actions were “worse than the misconduct of any prior president,” including Richard Nixon during Watergate.
He said: “If Congress fails to impeach here, then the impeachment process has lost all meaning. And, along with that, our Constitution’s carefully crafted safeguards against the establishment of a king on American soil.
“This president has attacked each of the Constitution’s safeguards against establishing a monarchy in this country. If this is not impeachable then nothing is impeachable.”