Former Vice-President Joe Biden says impeachment proceedings should begin against Donald Trump if he does not comply with an inquiry into a call he made with the Ukrainian president.
The Democratic presidential front-runner is the latest top party member to join impeachment calls.
Democrats accuse President Trump of improperly pressuring the Ukrainian president to undermine Mr Biden.
No US president has ever been removed from office by impeachment.
Impeaching Mr Trump “would be a tragedy”, Mr Biden said. “But a tragedy of his making.”
President Donald Trump has acknowledged withholding US aid to Ukraine and pressing his Ukrainian counterpart to investigate his would-be White House challenger Joe Biden.