'You have to ask Rudy': 5 key things to know about Rudy Giuliani’s involvement in the Ukraine affair

Once known as “America’s mayor,” Rudy Giuliani has emerged as a central figure in the impeachment inquiry into whether President Donald Trump abused his power as a sitting president to benefit his political campaign.

Here are five key things to know about Giuliani’s involvement in the Ukraine affair:

Giuliani engaged a foreign power as Trump’s personal lawyer

Giuliani doesn’t work on behalf of U.S. taxpayers as a diplomat or other government employee; he’s the president’s personal lawyer.

That’s why it was so astonishing last May when Giuliani told The New York Times that he planned to travel to Ukraine to press the government on matters that some Trump’s allies thought would help Trump’s re-election bid.

The goal, he said, was to find information that would be “very, very helpful to my client, and may turn out to be helpful to my government.”

Giuliani said he specifically wanted Ukraine’s new government to find evidence that corrupt politicians there had interfered in the 2016 election on behalf of Democrat Hillary Clinton. (U.S. intelligence has determined that Russia, not Ukraine, interfered in the U.S. election -- to benefit Trump, not Clinton.)

Giuliani also wanted the foreign power to look into potential conflicts of interest involving Democrat presidential candidate and former Vice President Joe Biden, and his son Hunter Biden, who served on the board a Ukraine gas company at the time. Both Biden and his son, Hunter, have denied any wrongdoing.

Giuliani told The New York Times in May that he wasn’t meddling in the upcoming election. “We’re meddling in an investigation, which we have a right to do,” he said.

Giuliani says he was acting on behalf of the president

Internal texts and documents suggest Giuliani was so deeply entangled in U.S. discussions with Ukraine, that senior State Department officials sought him out repeatedly to keep him updated.

“The key to changing the President's mind on Ukraine was Giuliani,” said Gordon Sondland, the U.S. ambassador to the European Union, in testimony to Congress.

And in his July phone call to Ukraine’s president, Trump made clear it was Giuliani who was acting on his behalf and not the former U.S. ambassador to Ukraine, Marie Yovanovitch, whom Trump called “bad news.”

“He was the mayor of New York City, a great mayor, and I would like him to call you. I will ask him to call you along with the Attorney General,” Trump said of Giuliani to Ukrainian President Volodymyr Zelenskiy.

“Rudy very much knows what’s happening and he is a very capable guy,” Trump added, according to a rough White House transcript of the call. “If you could speak to him that would be great.”

Giuliani earned $500K from a man accused of funneling foreign money into US campaigns.

Two associates of Giuliani – both Soviet-born, Florida-based businessmen – were arrested last week at a Washington-area airport with one-way tickets.

The men are accused of breaking campaign finance laws by funneling money from an unnamed foreign national – identified in the indictment only as a “Russian citizen and businessman” – into political contributions to specific U.S. political campaigns. Both men have pleaded not guilty.

When asked whether he knew the men, Trump said he wasn’t sure. “You have to ask Rudy,” he told reporters.

The business relationship between Giuliani and the two men is now the subject of an ongoing investigation conducted by federal authorities in New York, ABC News reported last Friday. And raising questions in particular is a $500,000 payment that Giuliani acknowledges that he received for work he did with a company co-founded by one of the men.

Giuliani’s influence remains outsize on other matters, too

According to a source with direct knowledge of the matter, Trump in 2017 urged then-Secretary of State Rex Tillerson to push the Department of Justice to drop a criminal case against one of Giuliani's clients -- an Iranian-Turkish gold trader accused of helping the Iranian government evade U.S. economic sanctions. Giuliani did not respond to a request to comment when reached by ABC News.

Separately, a former senior administration official tells ABC News that Giuliani urged Trump to extradite a Turkish cleric living in exile in the U.S.

The former senior official tells ABC that White House officials stepped in and blocked any actions, telling Giuliani that the Turkish government should go through proper channels for extradition requests. Giuliani declined to comment, citing attorney-client privilege.

Giuliani’s business relationships are under investigation

With Giuliani’s business relationships with Lev Parnas and Igor Fruman under investigation, the question is what happens next and how his dealings will impact the administration.

According to the Times, former national security adviser John Bolton told another White House staffer that Giuliani was a “hand grenade who’s going to blow everybody up.”

Giuliani has denied any wrongdoing and insisted that others at the White House didn’t have the “evidence” he did of “Ukrainian collusion.”

It’s not the first time a personal lawyer to Trump was scrutinized by law enforcement. Michael Cohen is now serving a three-year prison sentence after pleading guilty to tax, bank and campaign finance crimes.

Unlike Cohen, Giuliani says he is not complying with a congressional subpoena.

Giuliani tells ABC News “if they enforce it, then we will see what happens.”

ABC News' John Santucci contributed to this report.