Hillary Clinton says she believes the US President Donald Trump will be impeached.
He's accused of abusing power by withholding aid for Ukraine unless the country investigated his political rival, Joe Biden - claims Mr. Trump denies.
Clinton, a former US Secretary of State and Presidential candidate, says the testimony presented so far in the inquiry is devastating.
She says there are similarities with past impeachment proceedings such as the Richard Nixon Case.
Hillary Clinton says the 'abuse of power, the obstruction of justice and the contempt of Congress in that case have already been seen in the proceedings against Donald Trump'.
The former US ambassador to Ukraine was the third person to testify publicly yesterday.
Appearing as a witness, Marie Yovanovitch described a "smear campaign" against her by Mr. Trump's allies, including his personal lawyer Rudy Giuliani, before she was fired.
In her opening remarks, she said: "These events should concern everyone in this room.
"Shady interests the world over have learned how little it takes to remove an American ambassador who does not give them what they want."
The daughter of immigrants who fled the former Soviet Union and Nazi Germany, she described a 33-year career, during which she served both Republican and Democratic presidents.
Recalling how she felt when she read comments made by Mr Trump denouncing her in a phone call with Ukraine's president, Ms. Yovanovitch said someone with her said: "the colour drained from my face".
She added: "Even now words fail me."