Well before the Donald Trump was sworn in as the 45th President of the United States, opponents started discussing how he could be removed from the highest office in the United States. With all the controversy that continues to surround his presidency less than four weeks in, this discussion will not be going away any time soon.

So how could Donald Trump be ousted as president? The most commonly discussed scenario is impeachment. But what is impeachment, and how does it work?

Impeachment is the process of removing public officials from office on the basis of their conduct. Impeachment, however, does not necessarily mean removal from office. It is the decision by a legislature to accuse an official of one or more offenses that warrant removal from office. A vote to impeach, if successful, then prompts a trial based on those charges. The individual is then removed from office if the legislature votes to convicts the individual.

The power to impeach a president is found in Article II, Section 4 of the U.S. Constitution: "The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."

The process to impeach the president begins in the United States House of Representatives. An investigation to determine if allegations against a president warrant charges, or "articles of impeachment", can be initiated by the House Judiciary Committee, or a simple majority vote of the full House (218 of the 435 members). The investigation is then conducted by the House Judiciary Committee.

Based on this investigation, the House then votes on whether to charge a president with "articles of impeachment". If a simple majority of the full House votes in favour of charging the president with at least one article of impeachment, the process then moves to the Senate for trial. At this point, the House has "impeached" the president, however, the president has not yet been removed from office.

The trial then takes place in the Senate chamber, with the chief justice of the Supreme Court of United States presiding over the trial. Members of the House act as, or appoint, prosecutors to present their case against the president. After the House has presented their case, and the president has presented his defence to the charges, a two-thirds majority vote (67 of the 100 senators) is required to convict and remove the president from office.

While the impeachment process does exist as a check on the power of the presidency, the likelihood that President Trump would be impeached is slim.

First, Trump would have to commit either treason, bribery, or another high crime or misdemeanor to warrant articles of impeachment. However, in the wake of a New York Times article alleging that Trump campaign aides had repeated contact with Russian intelligence officials, this improbable scenario is seeming less and less far-fetched. Democratic House members are already taking aim at the Trump administration saying that if the administration or campaign have been conspiring with Russia, "that's the very definition of treason." And if it turns out that Trump himself had some level of involvement in a conspiracy with the Russians, it could lead to a charge of treason.

Then, a majority of the Republican-controlled House, and a two-thirds majority of the Republican-controlled Senate are still needed to impeach and convict Trump. In the partisan climate that is Washington, D.C., members of Congress often put party loyalty above all and may not be willing to crucify a member of their own political party, even if there is evidence that he has committed a crime. It could be damaging to the Republican "brand" and cost the party dearly in presidential, congressional, and state elections, if Trump were to be impeached.

While the Senate has never convicted and removed a president from office, the House of Representatives has impeached two past presidents: Andrew Johnson and Bill Clinton. Andrew Johnson was charged in 1868 with eleven articles of impeachment for firing his Secretary of War and issues surrounding his post-Civil War Reconstruction policy. Bill Clinton was charged in 1998 with two articles of impeachment for perjury and obstruction of justice relating to his relationship with White House intern, Monica S. Lewinsky. When Richard Nixon resigned in 1974, he was days away from being charged by the House with three articles of impeachment for his alleged involvement in the Watergate burglary scandal.

Beyond impeachment, the other, less discussed, scenario that could remove the president from power is the use of the 25th Amendment to the U.S. Constitution.

Section 4 of the 25th Amendment outlines what happens if the president is unable to discharge the powers and duties of his office. The amendment was passed by Congress on July 6, 1965, and ratified 584 days later on Feb 10, 1967, in the wake of uncertainty surrounding presidential succession, and responding to presidential disabilities.

Under the 25th amendment, if the vice president and the majority of the president's cabinet deem the president unable to discharge the powers and duties of his office, they can advise the House and Senate of this inability, and the vice president then takes over as acting president.

The president, however, can notify the House and Senate at any time that he is able to serve, and can then resume the duties of his office.

But if the vice president and a majority of the cabinet disagree that the president is fit to return to his office, the House and Senate then have to vote on whether the president should be removed. In order to remove the president from office against his will, a two-thirds majority is required in both the House and Senate.

Like impeachment, there is such a remote chance that the 25th amendment would ever be used to remove President Trump from power against his will. George Washington University's Graduate School of Political Management professor Gary Nordlinger believes, "Elvis Presley has a better chance of being found alive than President Trump being removed by the 25th Amendment".

The 25th amendment has only ever been used by a president to remove himself from power for short periods of time. In 1985, President Ronald Reagan temporarily removed himself from power for approximately eight hours during a surgical procedure—making Vice-President George H.W. Bush the acting president. Then in 2002, and again in 2007, President George W. Bush temporarily handed power to Vice-President Dick Cheney while he underwent a colonoscopy.

Removing the president through the 25th amendment is more likely to be seen in a movie, television show, or novel, than in reality. But if those closest to the president, in their judgment, believe that President Trump is no longer of sound mind to be president, the 25th amendment provides the power to do something about it.

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