New York: During the tumultuous first three years of his presidency, Donald Trump has experienced some damaging days. Friday (Saturday AEST) was up there with the worst.

The second day of public hearings in the impeachment inquiry against Trump began with gripping and emotionally moving testimony from an impeccable witness.

President Donald Trump on Friday. AP

As she was speaking, Trump jumped onto Twitter to lambast that witness - a move Democrats seized upon as a potential article of impeachment.

Then it was announced that one of the President's most longstanding allies, infamous "dirty trickster" Roger Stone, had been found guilty of multiple crimes and was headed to jail.

All that was before lunchtime.

The public phase of the impeachment process began on Wednesday (Thursday AEDT) with damning but not necessarily captivating testimony from two veteran US diplomats Bill Taylor and George Kent.

Former US ambassador to Ukraine Marie Yovanovitch provided emotionally compelling testimony. Bloomberg

They brought important insights about the Trump administration's efforts to pressure Ukraine to launch a public investigation into the family of his potential 2020 rival Joe Biden.

Based on the strength of her testimony on Friday, it's arguable that Democrats should have started with former US ambassador to Ukraine Marie Yovanovitch.

Her testimony delivered something more human: a compelling personal story of a career public servant and corruption fighter smeared and eventually thrown overboard by an American president and his allies.

George Kent, deputy assistant US secretary of state, left, and Bill Taylor, acting US ambassador to Ukraine, prepare to give testimony on Wednesday. Bloomberg

She presented such a sympathetic figure that even Fox News viewers could not avoid it.

"If you are not moved by the testimony of Marie Yovanovitch today, you don't have a pulse," Fox anchor Chris Wallace said.

During the first day of public hearings Trump was restrained in his comments, allowing the White House to say he was busy governing while the Democrats were tearing him down.

But Yovanovitch clearly got under his skin.

As she was testifying about the smear campaign against her, Trump tweeted: "Everywhere Marie Yovanovitch went turned bad.

"She started off in Somalia, how did that go?

"Then fast forward to Ukraine, where the new Ukrainian President spoke unfavourably about her in my second phone call with him."

Adam Schiff, the Democratic Chair of the House Intelligence Committee, immediately grasped the tweet's potential as an article of impeachment.

"We take this kind of witness intimidation and obstruction of inquiry very seriously," he said.

Even loyal Trump supporters said it was a bad look for the President to attack a witness while she was testifying.

Roger Stone, who advised Trump's 2016 campaign, was found guilty of seven criminal counts of lying to Congress, obstruction and witness tampering.

Stone served as a crucial link between the Trump campaign and WikiLeaks, which published a trove of hacked Hillary Clinton emails during the 2016 election.

He has so far remained steadfastly loyal to the President. The question now, as he faces many years in jail, is whether that remains the case.

By early afternoon at the Capitol Building, Republicans on the House Intelligence Committee had given up questioning Yovanovitch and were reading aloud news articles on unrelated topics.

They pointed out that Yovanovitch was removed from her post before Trump's infamous phone call with the Ukrainian President and before US military aid to Ukraine was withheld. That's true - as was their argument that the President can recall ambassadors as he chooses.

Her testimony alone would not be sufficient grounds for impeachment. But it represents an important piece of the puzzle, demonstrating that the warping of US foreign policy went far beyond one phone call in July.

Roger Stone, a longtime Republican provocateur and former confidant of President Donald Trump, is headed for jail. AP

The public hearings continue next week with Gordon Sondland, the US ambassador to the European Union, set to be the key witness. The Trump appointee amended his initial private testimony to confirm that there was a direct link between military aid for Ukraine and an investigation into the Bidens.

There is still no sign that Republicans are abandoning Trump. But Democrats ended the week with a spring in their step and Yovanovitch with a standing ovation from the public gallery.