How does impeachment work? Which US presidents have been impeached? A guide to US impeachment process.

Amid controversy over a summer phone call between US President Donald Trump and the new leader of Ukraine, House Speaker Nancy Pelosi ordered a formal impeachment inquiry of the US president in September.

“No one is above the law,” she said as she announced the House was moving forward with an official impeachment inquiry of Trump.

Pelosi, a Democrat, had earlier resisted calls to begin impeachment proceedings, urging restraint as House committees aggressively investigated Trump, a Republican, through subpoenas of witnesses and documents.

But following allegations that Trump pressured the leader of Ukraine to investigate the family of former Vice President Joe Biden, who is vying for the 2020 Democratic presidential nomination, pressure from inside the Democratic caucus for an impeachment inquiry mounted.

The House held hours of hearings with dozens of witnesses and produced hundreds of pages of reports.

On Wednesday, the House voted to impeach Trump on two articles of impeachment: abuse of power and obstruction of Congress.

As Trump becomes only the third US president in history to be impeached, here is what you need to know about the process:

1. What is impeachment in the US political system?

The founders of the United States included impeachment in the US Constitution as an option for removal of presidents by Congress.

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Impeachment, a concept in English common law, was one of the more hotly debated points during the constitutional convention of 1787 in Philadelphia. Delegates agreed that presidents could be removed if found guilty by Congress of “treason, bribery or other high crimes and misdemeanors”.

The sole authority under the Constitution to bring articles of impeachment is vested in the House of Representatives where proceedings can begin in the Judiciary Committee. If the House approves articles of impeachment, or “impeaches” a president, he or she would then be subject to trial in the US Senate.

2. On what grounds can a president be impeached? How does impeachment work?

Under the Constitution, the president, vice president and “all civil officers of the United States” can be removed from office for “treason, bribery, or other high crimes and misdemeanors”.

To begin impeachment proceedings, a House member can introduce an impeachment resolution, or the entire House can vote to initiate an investigation into whether there are grounds for impeachment. The House Judiciary Committee or a special committee will then investigate. The panel votes on whether to bring a vote to the full House.

Impeachment in the 435-member House must be approved with a simple majority.

If the House votes to impeach, the matter moves to the Senate, where a trial is held.

3. What are articles of impeachment?

Articles of impeachment are formal charges. In the case of Trump, the House considered two articles of impeachment: Abuse of power and obstruction of Congress.

Democrats accuse Trump of pressuring Ukraine to open an investigation the president’s political rival and former vice president, Joe Biden, who is also a frontrunner in the 2020 Democratic presidential race. They also charge that the president obstructed their investigation by refusing to comply with subpoenas and directing members of his administration to do the same.

The impeachment inquiry, launched in September following a whistle-blower complaint, was centred on a July 25 phone call during which Trump asked Ukrainian President Volodymyr Zelenskyy to open an investigation into Biden and his son, Hunter, who had served on the board of a Ukrainian gas company. There has been no evidence of wrongdoing by the Bidens.

Trump also wanted Zelenskyy to investigate a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US presidential election.

At the time of the call, the Trump administration was withholding nearly $400m in Congress-approved military assistance from Ukraine.

Citing testimony by current and former US officials, Democrats also accuse Trump of leveraging a White House meeting that Zelenskyy wanted in exchange for the investigations.

4. What does Trump say?

Earlier in the inquiry, Trump asserted that a formal impeachment inquiry by House Democrats would be “positive for me”.

He said the country is “doing the best it’s ever done” and that Democrats are going to lose next year’s elections if they pursue impeachment.

But since then, Trump has lashed out at Democrats, calling the inquiry “witch-hunt garbage”.

Trump maintains that he did nothing wrong, and that the call between him and the Ukrainian leader was “perfect”.

Trump on Tuesday sent a sharply-worded letter to Speaker Nancy Pelosi, the top Democrat in the House, expressing his “strongest and most powerful protest” against impeachment. He said he was innocent of all the charges, and if Democrats proceeded they would be “declaring open war on American democracy”.

“I have no doubt the American people will hold you and the Democrats fully responsible in the upcoming 2020 election,” Trump said. “They will not soon forgive your perversion of justice and abuse of power.”

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3. Now that Trump is impeached, when will the Senate hold a trial?

A two-thirds majority vote is required in the 100-member Senate to convict and remove a president from office.

The Senate is made up of 53 Republicans, 45 Democrats and two independents who caucus with the Democrats. At least 20 Republicans would have to vote with all Democrats and the two independents to remove the president.

A trial in the Republican-controlled Senate is expected in January. No date has been set, but Senate Majority Leader Mitch McConnell said this week that a trial would be the Senate’s “first order of business”.

Senate Democrats proposed a trial plan that would see proceedings begin on January 6. Presentations by House managers, who would effectively work as prosecutors, would begin on January 9 under this plan. It’s unlikely that Senate Republicans would agree to the Democrats’ exact proposal.

McConnell will likely announce details of a Senate trial after the House vote this week.

During the trial, the chief justice of the US Supreme Court presides over the trial.

4. Who would become president if Trump was impeached and removed? What would happen to Trump?

An unlikely Senate conviction that removed Trump from office would automatically elevate Vice President Mike Pence to become president, completing Trump’s term, which ends on January 20, 2021.

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Criminal charges cannot be brought against a sitting president, however, the Constitution does allow for separate criminal charges once a president is removed.

5. Which presidents have been impeached?

Other than Trump, only two other US presidents have ever been successfully impeached and in neither instance was the president removed from office. Andrew Johnson was impeached in 1868 in the tumultuous aftermath of the American Civil War; and Bill Clinton in 1998 for issues including his relationship with White House intern Monica Lewinsky. Both times, the House approved formal charges and impeached the president, only to have the Senate fail to convict and remove him.

The House Judiciary Committee in 1974 voted to recommend impeachment accusing another president, Richard Nixon, of planning to obstruct an investigation in the Watergate scandal. Before the full House could vote on impeachment, Nixon became the only US president ever to resign.