Hilary Clinton has said she believes US President Donald Trump will be impeached.
Mr Trump is accused of abusing power by withholding aid for Ukraine, unless the country investigated his political rival Joe Biden.
The former US Secretary of State and Democratic presidential candidate said the testimony presented so far in the inquiry is "devastating" against Mr Trump.
On Friday, the country's former ambassador to Ukraine Marie Yovanovitch said she thought corrupt Ukrainians helped her downfall.
Ms Yovanovitch said: "What continues to amaze me is that they found Americans willing to partner with them and, working together, they apparently succeeded in orchestrating the removal of a US ambassador.
"How could our system fail like this? How is it that foreign corrupt interests could manipulate our government?
Former US Ambassador to Ukraine, Marie Yovanovitch, testifies before the House Intelligence Committee on Capitol Hill in Washington | Image: Susan Walsh/AP/Press Association Images"Which country's interests are served when the very corrupt behaviour we have been criticising is allowed to prevail?
"Such conduct undermines the US, exposes our friends, and widens the playing field for autocrats like [Russian] President Putin.
"Our leadership depends on the power of our example and the consistency of our purpose. Both have now been opened to question."
Mr Trump also attacked Ms Yovanovitch on Twitter as she testified.
"I can't speak to what the president is trying to do, but I think the effect is to be intimidating", she said.
The head of the US House Intelligence Committee, Adam Schiff, told her: "Well, I want to let you know ambassador that some of us here take witness intimidation very, very seriously".
Earlier this week, the inquiry also heard from William Taylor - the top US diplomat in Ukraine.
Mrs Clinton, whose husband Bill was impeached during his presidency, said there are similarities with past impeachment cases.
"I think that the testimony that was already presented is devastating.
"These are not partisan people at all, they are public servants - two very experienced diplomats who were just reciting the facts as they knew them: what they had seen, what they had heard and why it mattered".
Mrs Clinton was also a lawyer on the impeachment staff who investigated Richard Nixon in the 1970s.
"I think that the Democrats in Congress who are running this process have been very careful [and] vigilant about not getting ahead of themselves, following the facts where they lead, and I will not be at all surprised if the House decides to impeach Donald Trump".
"I know from the experience with Richard Nixon - the abuse of power, the obstruction of justice, the contempt of Congress that were the allegations against him - we've already seen evidence of.
"It's pretty simple: our constitution talks about not only high crimes and misdemeanour, but also bribery.
"And bribery doesn't have to just be person to person, it can be nation to nation in the sense that a president would abuse his office to try to coerce the president of another country in desperate need of military assistance to protect his country against Russian aggression: that he would not get that assistance unless he manufactured information that could be used against the president's political opponents.
"That is exactly the kind of abuse that the people who wrote our constitution were most worried about".
