President Donald Trump seethed on Monday as Democrats in the U.S. House of Representatives prepared to enter a crucial new phase — the first public hearings — in their impeachment inquiry centered on his request that Ukraine investigate political rival Joe Biden.

On Wednesday and Friday, U.S. diplomats William Taylor, George Kent and Marie Yovanovitch are due to detail in public their concerns, previously expressed in testimony behind closed doors, that Trump and his administration sought to tie $391 million in security aid to Ukraine to an investigation of the former U.S. vice president and his son Hunter Biden.

The public testimony before the House Intelligence Committee will be carried by major broadcast and cable television networks and is expected to be viewed by millions of people, as Democrats seek to make the case for Trump's potential removal from office.