Impeachment: how does it work and what happens next?

The House speaker, Nancy Pelosi, has announced a formal impeachment inquiry into Donald Trump, making him the fourth president in US history to face such an extraordinary investigation.

Related: Pelosi announces impeachment inquiry into Trump over Ukraine scandal

All three precedent cases resulted in impeachment or in the resignation of the president.

What is happening?

After months of pressure from a significant portion of her caucus to greenlight impeachment proceedings, Pelosi finally came around on Tuesday, after 24 hours in which more than 30 House Democrats joined the call to open an impeachment inquiry.

The surge happened after Congress learned of a whistleblower report filed by a member of the intelligence community focusing on a conversation or conversations that Trump had with the president of Ukraine. The conversations reportedly involved the former vice-president Joe Biden, Trump’s potential 2020 opponent. Democrats allege that Trump delayed military aid and pressured Ukraine for information about Biden. Trump denies wrongdoing.

Related: Why is the Trump-Ukraine whistleblower scandal so serious?

How does the procedure work?

Pelosi said that the six congressional committees currently investigating potential impeachable offenses by Trump would continue to do so. Based on their findings, the judiciary committee could draft and approve articles of impeachment against Trump, which then would go to the House floor for a vote. If the articles are passed, the Senate will hold a trial on the matter, with a two-thirds majority required to convict and remove the president.

As things stand,Trump’s removal from office would thus require the partisan defection of about 20 Republican senators.

What’s next?

It appears that a lot of additional information about the Ukraine affair is about to come to light. The House intelligence committee has announced that the whistleblower wishes to testify, and could do so as early as this week, meaning that the public could soon learn the details of the allegations against Trump – and see what evidence there is.

In a rare unanimous vote, the Senate on Tuesday approved a resolution calling on the White House to turn over the whistleblower’s report.

Trump tweeted on Tuesday that he would release a transcript on Wednesday of a phone call with the Ukrainian president, Volodymyr Zelenskiy, despite earlier vows by Trump’s lawyer Rudy Giuliani that no such transcript would be released.

What might Trump be impeached for?

That’s an important question. Trump could conceivably face impeachment for a wide range of alleged misconduct including personally profiting from the presidency, violating campaign finance laws, inappropriately diverting funds to build a border wall or dangling pardons to induce lawbreaking.

But some commentators have warned that Congress should focus on a narrow group of alleged offenses, not only to avoid the perception of a partisan expedition but also to avoid setting a precedent that could radically expand the scope for future impeachment inquiries.

The editors of the Lawfare blog have argued for articles of impeachment that would avoid areas of policy disagreement and be tied only to Trump’s conduct while in office, for which there is the most strong evidence, and on which the constitution is most plain.

They recommend five articles: obstruction of justice and abuse of law enforcement institutions and personnel; prosecution of political opponents; abuse of foreign policy authorities; efforts to obstruct or impede congressional investigations; and lying to the American public.

What are the political implications?

Anybody’s guess. The conventional wisdom is that the impeachment of Bill Clinton in 1998 backfired on Republicans, who saw Clinton’s popularity rise while they suffered heavy losses in the subsequent election. But the causality there is unclear and we don’t have the counterfactual.

Pelosi’s longstanding reservations about impeachment, which she made clear repeatedly after the Mueller report was released in redacted form, appeared to be rooted in concerns that impeachment proceedings would make Democrats seem unfocused on the work of governing and possibly help Trump’s re-election bid. She might have a point: “On average, in all polls since the start of 2017, 38.5 percent of the public favored impeachment and 55.7 percent opposed it,” according to FiveThirtyEight.

But for many Democrats, such concerns have gradually paled in comparison with Trump’s alleged criminality.

Who has been impeached?

Two presidents, Bill Clinton (1998) and Andrew Johnson (1868). (Congress may also impeach judges.) Articles of impeachment were passed against Richard Nixon by a congressional committee, but Nixon resigned before the House of Representatives could vote on the matter, meaning that technically he was not impeached.

Both Johnson and Clinton were impeached in the House but then acquitted in the Senate and remained in office.

What can a president be impeached for?

“Treason, bribery, or other high crimes and misdemeanors”, the constitution says. Needless to say, there’s debate over what all those terms mean.

Johnson was charged with breaking the law by removing the US secretary of war, which, in the aftermath of the civil war, was not his decision as president to make. Clinton was charged with obstruction of justice and with perjury, for allegedly lying under oath to a federal grand jury about his affair with Monica Lewinsky.

Had Nixon not resigned, he might have been convicted in the Senate on one of three charges: obstruction of justice, abuse of power or defiance of subpoenas. In any case, President Gerald Ford, who was Nixon’s vice-president and who succeeded him, pardoned Nixon of any crimes a month after Nixon resigned.

How long do impeachment proceedings take?

There isn’t much precedent, but the Clinton case proceeded through Congress relatively quickly, in about three months. That example may be misleading, however, owing to the years-long investigation of Bill and Hillary Clinton, including the Lewinsky affair, by the special prosecutor Kenneth Starr, which preceded it. Starr handed his report and research to the House judiciary committee, which therefore had no need to conduct a time-consuming investigation of its own.