US House Speaker Nancy Pelosi opened the debate on impeachment ahead of the historic vote.
In her opening statement, the top Democrat said “the president is an ongoing threat to our national security.”
The House of Representatives, the lower chamber of Congress, is to vote on whether to impeach President Trump.
The president is accused of abuse of power and obstruction of Congress related to his dealings with Ukraine. Mr Trump denies any wrongdoing.
Read more: US House ready for historic vote