United States (U.S.) President Donald Trump survived the two-year Russia election meddling probe intact, but the revelation that he asked Ukraine to dig up dirt on potential 2020 election campaign rival Joe Biden has now exploded into a scandal threatening his presidency.

This is a look at the key figures in the fast-moving drama, which was triggered by Trump's request in a July phone call with Ukraine's President Volodymyr Zelensky that he investigate Biden and his son Hunter.

Once 'America's mayor'

Many Americans remember Rudy Giuliani, a former federal prosecutor, as the competent and reassuring mayor of New York City during the September 11 attacks.

Now, as a fast-talking, media-fixture lawyer for Trump, he has turned up at ground zero of the Ukraine scandal amid revelations that he was Trump's roving point man for a drive to persuade Zelensky to launch a corruption investigation into the Bidens.

Speaker of the House

Nancy Pelosi, a California Democrat serving as speaker of the House of Representatives, is widely considered one of the cleverest politicians in Washington.

Since Democrats took over the House in November of last year, congressional committees have been investigating Trump's finances, his ties with Russia and other alleged misconduct. But Pelosi resisted calls from within her party to pursue impeachment, instead opting to play a waiting game.

However, after the White House released a rough transcript of the July 25 call, in which Trump is clearly quoted as asking the Ukrainian president for "a favor" right after discussing how much America helps Ukraine and wants reciprocity, Pelosi pulled the trigger on impeachment.

"The president must be held accountable. No one is above the law," Pelosi said Tuesday as she announced a formal impeachment probe.

Shadowy whistleblower

All of this started when an anonymous whistleblower learned of what Trump said to Zelensky in the July 25 call and wrote a detailed complaint about it, sending the memo to the inspector general for the US intelligence community.

After delays that the Democrats criticised as stonewalling, the Trump administration made it available to Congress, and it was eventually made public.

Critics of Trump called it stunning -- a show of him blatantly harnessing the power of the US government to recruit a foreign power to help him win re-election. Trump allies dismiss the complaint as containing no real evidence of wrongdoing.

The identity of the whistleblower, whom an irate Trump, has excoriated as a spy who should get old-style punishment, remains unknown.

The New York Times has reported the person is a male CIA officer who used to be posted to the White House and is an expert on Europe and politics in Ukraine.

Face of the probe

Adam Schiff, another California Democrat, is the soft-spoken and seemingly unflappable chairman of the House Intelligence Committee. He is serving as the leader of the impeachment probe, which involves three committees.

After the whistleblower complaint was made public, Schiff said it showed a clear quid pro quo: Trump promising to release hundreds of millions of dollars in suspended military aid for Ukraine as it wages war with Russian-backed separatists, in exchange for Zelensky opening a probe of the Bidens.

"This is how a mafia boss talks," Schiff said.

He added, imitating Trump, "'What have you done for us? We've done so much for you but there's not much reciprocity. I have a favour to ask you.'"

Ex-comic seeks US support-

Zelensky is a 41-year-old actor and comedian who was elected President of Ukraine in April of this year, inheriting the war his country is fighting against the separatists.

In the transcript of the July 25 conversation with Trump, he seems eager to win the support of the Trump administration.

And he flatters the US president effusively, telling him he learned a lot from Trump's campaigning style and once stayed at a Trump hotel, in New York.

Trump tells the young leader he will receive him at the White House.

The two men met Wednesday on the sidelines of the UN General Assembly and afterward, speaking to reporters with Trump, Zelensky thanked him for the invitation to Washington.

“You invited me. But I think -- I'm sorry, but I think you forgot to tell me the date,” he said. (AFP)

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