Why is the Trump-Ukraine whistleblower scandal so serious?

Donald Trump is facing on one of the most perilous periods of his historically perilous presidency.

Related: Impeachment talk intensifies over Trump's call with Ukraine president

Reports that he urged a foreign leader to investigate Joe Biden, seen as his main rival in the 2020 presidential election, have rocked Washington and intensified calls for Trump to be impeached.

Trump is set to meet Ukraine’s president, Volodymyr Zelenskiy, the leader in question, in New York on Wednesday.

On Thursday, Democrats on the House intelligence committee will grill Joseph Maguire, the acting director of national intelligence (DNI). Maguire has refused to release details of a whistleblower’s complaint said to relate to a phone call between Trump and Zelenskiy, despite intensifying calls from Democrats and even some Republicans.

How did this begin?

A whistleblower, working in US intelligence, filed a formal complaint in August. The unnamed whistleblower is said to have seen or heard something that raised “urgent concern”.

Reporting has since revealed that the complaint was based on a series of events, including a 25 July call between Trump and Zelenskiy. Complaints of this nature are usually reported to Congress within seven days. Maguire, who was appointed by Trump, has refused to do so.

What happened on the call?

Numerous news outlets have reported that Trump pressured Zelenskiy to investigate Joe Biden’s son, Hunter Biden, who served on the board of a Ukrainian energy company. According to the Wall Street Journal, Trump asked Zelenskiy eight times to investigate Hunter Biden.

On Sunday, Trump confirmed he discussed the Bidens with Zelenskiy, and accused the pair – without offering any evidence – of corruption.

“The conversation I had was largely congratulatory, was largely corruption, all of the corruption taking place,” he said. “Was largely the fact that we don’t want our people, like Vice-President Biden and his son, [contributing] to the corruption already in the Ukraine.”

Why is Ukraine involved?

There is no evidence of any wrongdoing by Joe Biden or Hunter Biden. The allegations relate to Biden Jr’s time on the board of a Ukrainian energy company, Burisma, which was owned by a businessman who was being investigated by Ukraine’s top prosecutor.

In 2016 the US and a number of other western countries pressured Ukraine to remove the prosecutor. Biden was vice-president at the time.

Trump’s push for the Bidens to be investigated relates to the prosecutor’s removal. Trump’s assertions appear to be politically motivated. Joe Biden is favorite to win the Democratic nomination for president and polls show him beating Trump comfortably.

Why is this so serious?

It is illegal for a political campaign to accept a “thing of value” from a foreign government. Democrats say an investigation into a political opponent – for which Trump appears to have been pushing – would amount to a thing of value.

It has also been suggested that Trump may have threatened to withhold military aid to Ukraine. About a month after the call, the US delayed $250m in such assistance. The money was released this month, after the existence of the whistleblower complaint became public.

On Monday, at the United Nations in New York, Trump said of the call: “It’s very important to talk about corruption. If you don’t talk about corruption, why would you give money to a country that you think is corrupt? … It’s very important that on occasion you speak to somebody about corruption.”

The following day, the president confirmed that he had withheld military aid – but claimed he did so over concerns the US was contributing more than its fair share compared with other countries.

Trump also continued to seek to cast doubt on the whistleblower’s motivations, tweeting: “Who is this so-called ‘whistleblower’ who doesn’t know the correct facts. Is he on our country’s side. Where does he come from?”

Will the Trump-Zelenskiy transcript be released?

Amid the growing calls for an impeachment inquiry, Trump on Tuesday tweeted that he had ordered the release of an unredacted transcript of the call the following day.

Democrats are arguing that in addition to the transcript of the call, the full whistleblower complaint should be released to Congress.

What are Democrats saying?

The House speaker, Nancy Pelosi, on Tuesday announced the US House of Representatives would begin a formal impeachment inquiry into the president. “The actions taken to date by the president have seriously violated the constitution,” Pelosi said in a formal address in Washington on Tuesday evening. “The president must be held accountable. No one is above the law.”

Pelosi had long resisted pressure from the Democratic rank-and-file to impeach Trump over his links to Russia. But the Ukraine revelations only intensified calls for impeachment.

Also on Tuesday, the Senate unanimously approved a Democratic resolution calling for the DNI to turn over the whistleblower complaint to Congress.

• This story was updated on Tuesday 24 September with Pelosi’s announcement and other political developments.