Hunter Biden had joined the board of Ukrainian energy company Burisma while his father was U.S. vice president. Trump has accused the Biden's of corruption without offering evidence.
They have denied wrongdoing.
The three professors chosen by the Democrats made clear they believed Trump's actions constituted impeachable offenses.
"The president's conduct described by the testimony embodies the (Constitution's) framers' concern that a sitting president would corruptly abuse the powers of office to distort the outcome of a presidential election in his favor," Harvard University law professor Noah Feldman told the panel.
'A KING ON AMERICAN SOIL'
University of North Carolina Professor Michael Gerhardt appeared to admonish Trump's Republican allies for "leaving unchecked a president's assaults on our Constitution." "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. No one, not even the president, is beyond the reach of our Constitution and our laws," Gerhardt said.