The Democrats announced a formal impeachment inquiry into Donald Trump on Tuesday, sparked by a mounting scandal involving the U.S. president, Democratic rival Joe Biden, former New York mayor Rudy Giuliani and Ukraine. 

What's Trump accused of?

The impeachment inquiry could cover a range of alleged misconduct currently being investigated by congressional committees, but the latest scandal entered the public eye with news of a whistleblower complaint about Trump's conversation with a foreign leader.

The U.S. president is alleged to have pressed the other leader to look into a case related to Joe Biden, a possible 2020 presidential opponent. However, the acting director of national intelligence (DNI) Joseph Maguire refused to release the complaint to Congress.

Newspapers soon linked the report to a conversation Trump had with Ukraine's new president, Volodymyr Zelensky, on 25 July — the day after Special Counsel Robert Mueller testified before Congress.

The Wall Street Journal reported that during the July 25 call, Trump pressed Zelensky eight times to work with Giuliani on an investigation into Hunter Biden, Joe Biden's son. The Washington Post reported that one week earlier the U.S. president had ordered to withhold 400 million U.S. dollars in aid to Ukraine.

Trump denies any wrongdoing or "quid pro quo," but admits mentioning Biden to Zelensky. He is accused of trying to pressure Ukraine to reopen an investigation against a political opponent using foreign aid money as leverage.

What's Biden's role?

Trump and his supporters, led by personal lawyer Giuliani, allege that as vice president, Joe Biden pressured Ukraine to dismiss a prosecutor responsible for a criminal investigation involving Burisima Holdings, a company Hunter Biden sat on the board of.

Giuliani claims that in 2016 the then vice president threatened to withhold 1 billion U.S. dollars in loan guarantees to force the sacking of prosecutor, Viktor Shokin, to protect his son.

Biden denies any impropriety and no evidence has emerged that he acted improperly. He did call for the removal of Shokin, who was accused of taking bribes, but as part of an international alliance.

What's Giuliani got to do with it?

The Washington Post on Wednesday documented a series of events that reveal tensions within the administration in the run-up to June 25 and cast Giuliani as the key player.

The former New York mayor, who has no official role in the Trump administration, has been leading the charge against Biden and has made several controversial TV appearances. His activities are likely to be closely scrutinized over the coming days.

What happens next?

The next week should see significant new information come to light.

Trump has authorized the release of a "complete, fully declassified and un-redacted transcript" of the July 25 call with Zelenskiy, and said that it will be made public on Wednesday. Reuters has reported that the document may be based on notes, rather than verbatim.

The whistleblower is expected to testify before Congress soon and the Senate has approved a resolution calling on the whistleblower's report to be released. Maguire, the acting DNI, who has so far prevented Congress from seeing the report, is due to appear before lawmakers on Thursday.

What's the impeachment process?

Six congressional committees currently investigating potential abuses by Trump will continue to do so, and based on their findings the judiciary committee can put articles of impeachment to a vote in the House. 

If passed, the Senate will hold a trial and a two-thirds majority will be needed to remove the president. Given the Republicans hold a 53-47 majority in the Senate, a two-thirds majority is highly unlikely.