U.S. House Speaker Nancy Pelosi has announced a formal impeachment inquiry of U.S. President Donald Trump following reports that he asked a foreign government to investigate a potential 2020 election opponent.

After allegations came out that Trump had pressed Ukraine’s leader to investigate former U.S. vice president Joe Biden -- a frontrunner for the Democratic presidential nomination -- Democrats began to call for impeachment proceedings to begin.

Pelosi had previously resisted the idea of impeachment, particularly after the Robert Mueller investigation, preferring to seek an electoral defeat of Trump. But on Tuesday she announced the House would move forward.

“This week, the president has admitted to asking the president of Ukraine to take actions which would benefit him politically,” Pelosi said Tuesday.

“The actions of the Trump presidency revealed the dishonorable fact of the president’s betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections. Therefore today, I’m announcing the House of Representatives is moving forward with an official impeachment inquiry.”

Her statement follows Trump’s announcement that he had authorized the release of a transcript of his phone call with Ukraine’s president. The Senate has unanimously passed a resolution calling for the full whistleblower complaint regarding the Ukraine controversy to be revealed to the House Intelligence Committee, not just the transcript of the call.

So what exactly happens when impeachment proceedings are announced?

What is impeachment?

Impeachment is the process to remove an official, and is enshrined in the U.S. Constitution. It can happen at the state or federal level. In the case of impeaching a sitting president, the House of Representatives can call for impeachment to begin if they believe that the president is guilty of “treason, bribery, or other high crimes and misdemeanors,” according to the Constitution.

There is no specific definition of a “high crime” included in the Constitution, but the concept is essentially aimed at preventing corruption by high-level officials. Although impeachment involves a trial, it’s not a criminal process, merely one to remove an official from office. Criminal charges being brought against an individual is a separate matter.

How does it happen?

An individual member of the House could simply introduce an impeachment resolution like any other bill, which would then be reviewed by a committee, likely the House Judiciary Committee. Or the House Judiciary Committee can first hold their own investigation and bring articles of impeachment against the president, and present that evidence to the House. (“Articles of impeachment” simply refers to the charges against a president).

Ultimately, the full House votes on whether or not to impeach. A majority vote is needed for the impeachment to pass.

If the House passes a vote to impeach, it doesn’t mean the president gets removed immediately. The case then moves to the Senate, where a trial is held.

The House appoints members to act as prosecutors in the Senate. The president would have defense lawyers to defend him. The chief justice of the Supreme Court would preside over the Senate trial.

It comes down to a final Senate vote. At least two thirds of the Senate needs to vote to impeach in order for the motion to pass.

impeachment

What happens afterwards if impeachment is successful?

The president would be immediately removed from office, and the vice president would take over until an election.

How often has impeachment been successful?

No sitting president has been removed from office due to impeachment so far, although impeachment proceedings have been brought against three different presidents.

Andrew Johnson in 1868 and Bill Clinton in 1998 both were ultimately acquitted and allowed to complete their presidential terms. Richard Nixon resigned in 1974 when impeachment proceedings were brought against him for his role in the Watergate scandal, preventing the House from impeaching him.

Johnson’s impeachment focused on whether the president could remove cabinet officers without Congress’s approval, while Clinton’s revolved around obstruction of justice and perjury.

The Senate is currently controlled by Republicans, which has been brought up by some Democrats in the past as a reason not to fight for impeachment proceedings against Trump, due to the fear that it would be unsuccessful and only bolster support for Trump among Republicans.

For now, the next step would be for the evidence to be presented before the House for a vote. House Judiciary Committee Chairman Jerrold Nadler has declared previously that his committee has already been conducting impeachment hearings, but the panel has been unable to get many key witnesses and documents from the Trump administration.

In response to Pelosi’s announcement, Trump tweeted, “They never even saw the transcript of the call. A total Witch Hunt!”

He followed it up with another tweet: “PRESIDENTIAL HARASSMENT!”