Trump impeachment: Here's how the process works Trump is the third president in history to be impeached.

Impeachment proceedings for Donald Trump are nearing an end today, with the president facing vote in the Senate on two articles of impeachment that were approved by the House -- abuse of power and obstruction of Congress.

While the process had long been discussed for Trump, Democrats’ efforts gained traction late last year following a whistleblower complaint about a controversial call between Trump and Ukraine President Volodymyr Zelenskiy was revealed that critics say included a quid pro quo for political gain.

Since taking control of the House in 2018, Democratic leadership had been wary to broach the subject of impeachment, even after the Mueller report was issued, which did not clear the president of obstruction of justice. But the Ukraine call and its fallout changed that calculus.

Trump has called the inquiry the continuation of a “witch hunt” that has dogged his presidency and Republicans members of Congress have attacked the process as a sham that disregards the president’s due process rights and impedes his ability to conduct foreign policy.

Here's how the process works:

The presidential impeachment process

An impeachment proceeding is the formal process by which a sitting president of the United States may be accused of wrongdoing. It is a political process and not a criminal process.

The articles of impeachment are the list of charges drafted against the president. The vice president and all civil officers of the U.S. can also face impeachment.

The process begins in the House of Representatives, where any member may make a suggestion to launch an impeachment proceeding. It is then up to the speaker of the House, as leader of the majority party, to determine whether or not to proceed with an inquiry into the alleged wrongdoing.

House Speaker Nancy Pelosi announced the formal opening of an impeachment inquiry on Sept. 24, citing what she called Trump’s “betrayal of his oath of office, betrayal of our national security, and betrayal of the integrity of our elections" and the full House voted on Oct. 31 to authorize the inquiry.

In Trump’s case, the House Intelligence Committee was tasked with the investigation and the matter was then turned over to the Judiciary Committee, which drafted two articles of impeachment -- for abuse of power and obstruction of Congress. The process is dictated entirely by the ruling party.

A simple majority of the members of the Judiciary Committee was needed for each article to proceed to a vote by the full House. The House Judiciary Committee consists of 24 Democrats and 17 Republicans, so 21 votes in favor were necessary.

The House approved both articles on Dec. 18 in mostly party-line votes. Both required simple majority votes, which is 50% plus one more vote (218), after which the president is impeached.

Trump now faces a vote on the articles in the Senate, which will decide whether he is to be removed from office. Conviction, at this point, is unlikely in the Republican-controlled Senate.

Justification for impeachment

When it comes to impeachment, the Constitution lists "treason, bribery, or other high crimes and misdemeanors," as justification for the proceedings, but the vagueness of the third option has caused problems in the past.

"It was a central issue with Andrew Johnson, and there was a question during Clinton's proceedings about whether his lie was a ‘low’ crime or a ‘high’ crime," Michael Gerhardt, a constitutional law professor at the University of North Carolina who authored a book on the impeachment process, told ABC News.

According to Suzanna Sherry, a law professor at Vanderbilt University who specializes in constitutional law, "nobody knows" what is specifically included or not included in the Constitution’s broad definition of “high crimes and misdemeanors.”

"It’s only happened twice and so the general thought is that it means whatever the House and the Senate think it means," Sherry said, and even if the House approves the article or articles of impeachment, the senators can choose to vote against the articles if they feel they are not appropriate.

What does an impeachment vote mean for a sitting president?

A president can continue governing even after he or she has been impeached by the House of Representatives. After Bill Clinton was impeached on Dec. 19, 1998,he finished out his second term, which ended in January 2001, during which time he was acquitted in a Senate impeachment trial. While Clinton continued governing, and the impeachment had no legal or official impact, his legacy is marred by the proceeding.

Where does the Senate come in?

The Senate is tasked with handling the impeachment trial, which is presided over by the chief justice of the United States. To remove a president from office, two-thirds of the members must vote in favor – at present 67.

If the Senate fails to convict, a president is considered impeached but is not removed, as was the case with both Clinton in 1998 and Andrew Johnson in 1868. In Johnson’s case, the Senate fell one vote short of removing him from office on all three counts.

While the Senate trial has the power to oust a president from office, and ban him or her from running for future office, it does not have the power to send a president to jail. Disqualification from holding office, a separate process, requires a simple majority vote, according to the Congressional Research Service.

"The worst that can happen is that he is removed from office, that's the sole punishment," Sherry said.

That said, a president can face criminal charges at a later point. Sherry points out that in the Constitution "the party convicted shall nevertheless be liable and subject to indictment, trial, judgment and punishment, according to law."

While presidential removal is unprecedented, the vice president would assume office under the 25th Amendment, which was ratified in 1967. Then the new president would nominate a new vice president who would have to be confirmed by a majority of both houses of Congress.

Past presidential impeachments

Aside from Trump, only Clinton and Johnson have been impeached and no U.S. president has ever been forced out of office.

Johnson faced impeachment in 1868 after clashing with the Republican-led House over the “rights of those who had been freed from slavery,” although firing his secretary of war, Edwin Stanton, who was backed by the Republicans, led to the impeachment effort. The articles of impeachment centered on the Stanton event, according to the Senate.

Clinton, whose impeachment was connected to the cover-up of his affair with White House intern Monica Lewinsky while in office, was 22 votes away from reaching the necessary number of votes in the Senate.

Richard Nixon faced three articles of impeachment related to the Watergate scandal, in which he allegedly obstructed the investigation and helped cover up the crimes surrounding the break-in.

Nixon faced articles of impeachment, meaning that the House Judiciary Committee approved three articles of impeachment in relation to the Watergate scandal, but didn’t let the process get any further.

But he didn’t let the process get any further, resigning before the House could impeach him.

Editor's note: This story was originally published in 2017 and has been updated periodically.