WASHINGTON — The congressional power to remove a president from office through the process known as impeachment is the ultimate check on the executive.
No president has ever been forced from the White House that way, though Richard Nixon resigned rather than have to face the prospect.
On Tuesday, House Speaker Nancy Pelosi announced "the House of Representatives is moving forward with an official impeachment inquiry" into President Donald Trump.
Congress gets the authority from the Constitution. The term "impeachment" is commonly used to mean removing someone from office, but it actually refers to the filing of formal charges.
The House impeaches. The Senate then holds a trial on those charges to decide whether the officer — a president or any other federal official — should be removed and barred from holding federal office in the future.
The House has impeached 19 people, mostly federal judges. Two presidents, Andrew Johnson and Bill Clinton, were impeached, but the Senate voted not to convict either of them.
Any member of the House can seek to start the process by proposing a resolution of impeachment. It would be referred to the House Judiciary Committee, which decides whether to investigate the allegations. If it does so, the committee would then vote on whether grounds for impeachment exist. A yes vote would report one or more charges, known as articles of impeachment, to the full House.
The Constitution provides that a president can be impeached for "treason, bribery, or other high crimes and misdemeanors." Treason and bribery are well understood, but the Constitution does not define "high crimes and misdemeanors."
Congress has identified three types of conduct that constitute grounds for impeachment, including misusing an office for financial gain. But the misdeeds need not be crimes. A president can be impeached for abusing the powers of the office or for acting in a manner considered incompatible with the office.
When Gerald Ford was a member of the House, he famously defined an impeachable offense as "whatever a majority of the House of Representatives considers it to be at a given moment in history." In other words, impeachment and conviction by Congress is a political punishment, not a criminal one.
Once the House Judiciary Committee acts, the full House votes. A simple majority of those present and voting is required to impeach. If the impeachment vote passes, House managers are appointed to serve as prosecutors in the Senate.
When a president has been impeached by the House, the Supreme Court's chief justice presides over the Senate trial. The office holder is entitled to defense lawyers, and each side can call witnesses and present evidence. When the case has been presented, the Senate meets in closed session to deliberate.
The Senate vote is conducted in open session, and a two-thirds majority is required for conviction. If the Senate votes to convict, the public official on trial is immediately removed from office. The Senate can conduct a separate vote on whether to disqualify the official from ever again holding federal office, which requires a simple majority.
The Senate's vote is final. The Supreme Court has said that because the Constitution gives the Senate "the sole power to try all impeachments," a conviction on impeachment cannot be appealed to any court.
In all of U.S. history, only eight people have been convicted after Senate impeachment trials, and all were federal judges.
Pelosi said the congressional committees currently investigating Trump — Oversight, Intelligence, Ways and Means, Financial Services, Foreign Services and Judiciary — would continue their probes under "that umbrella of impeachment inquiry." House Majority Leader Steny Hoyer said "we will continue to pursue the facts and follow them wherever they lead — including to articles of impeachment."
How long that process will take is unclear, but a source told NBC News that Pelosi told lawmakers they need to move expeditiously. "The president must be held accountable," she said in remarks after the meeting. She accused the president of violating his oath of office, national security and "the integrity of our elections" after his admission that he asked "the president of Ukraine to take actions which would benefit him politically."
In September, the Washington Post reported that a member of the U.S. intelligence community had filed a whistleblower complaint over Trump's communications with a foreign leader. The newspaper went on to report that Trump had urged Ukrainian President Volodymyr Zelensky to investigate business dealings of former Vice President Joe Biden's son, Hunter Biden, in order to dig up dirt on a leading 2020 Democratic contender.
The younger Biden worked for a Ukrainian gas company from 2014 to 2019, and the president has argued that his father acted corruptly to benefit his son. There is no evidence of this.
The president's call came days after he froze an $391 million aid package for the country, and Democrats allege it is a sign that the president abused the presidency to try and bolster his reelection chances by seeking the help of a foreign government.