Washington: Donald Trump's efforts to pressure Ukraine to investigate his potential 2020 rival Joe Biden constituted bribery and his misconduct was worse than any other president in US history, constitutional experts have testified to Congress.

The House Judiciary Committee, which is responsible for drafting articles of impeachment, began hearings on Wednesday (Thursday Australian time) with evidence from four scholars on whether Trump's behaviour fits the standard for impeachment outlined in the US constitution.

The constitution says that a president can be impeached and removed from office for committing "treason, bribery, or other high crimes and misdemeanours".

The three scholars selected by Democrats on the committee said that Trump's behaviour merited impeachment while the sole scholar selected by Republicans said that the case for impeachment had not been made.