Fifty years have passed since President Richard Nixon sent a letter on August 9, 1974 to then-Secretary of State Henry Kissinger, resigning the presidency under imminent threat of impeachment by the House of Representatives. Had he not resigned, he would almost surely have become only the second president in U.S. history to have been impeached since Andrew Johnson in 1868.

Half a century later, however, impeachment has become not a measure of last resort but a political weapon—one at odds with its purpose in the Constitution.

In addition to Johnson, who missed being convicted in his Senate trial by only one vote, two other presidents, Bill Clinton and Donald Trump, have been impeached but were also not convicted in their Senate trials. Only one president was impeached in the first 200 years of our history. Yet in the last 50 years, two presidents have been impeached (one twice), and a third president just missed being impeached because he resigned.

Why this increase of impeachments against sitting presidents? To understand that, one must look at the impeachment clause and what the Framers intended it to be—and not to be—used to remedy.

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The Constitution provides that 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 House has the “sole power of impeachment,” which requires a simple majority vote, and the Senate has the “sole Power to try all Impeachments,” which requires a two-thirds’ vote for conviction.

The House has impeached 21 officials, and the Senate held an impeachment trial for 16 of them, convicting eight—all federal judges.

Nixon was sworn in on January 20, 1973, after a landslide re-election in which he captured 49 states and 61 percent of the popular vote. Just 10 days later, two men were convicted of burglarizing the Democratic National Committee headquarters in the Watergate hotel complex the previous June (four others had already pleaded guilty), and the political dominoes began to fall.

Less than a month after Nixon took office, the Senate voted unanimously to create a select committee to investigate the Watergate break-in. Witnesses discussed Nixon’s “enemies list,” secret White House tape recordings, and connections between Nixon, his inner circle, and the Watergate burglars.

On February 6, 1974, the House voted 410-4 to authorize the Judiciary Committee to investigate whether sufficient grounds existed to impeach Nixon. On March 2, a federal grand jury indicted seven former Nixon administration officials, including Attorney General John Mitchell, on charges connected to the Watergate break-in, including obstruction of justice, perjury and lying to the FBI.

After an in-depth investigation and a few days after the Supreme Court unanimously ordered Nixon to turn over White House tapes of recorded conversations, the House committee voted 28-10 to approve three articles of impeachment for obstruction of justice, abuse of power and contempt of Congress.

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Remaining congressional support for Nixon disappeared when, on August 5, he released a transcript of another taped conversation that showed his complicity in the Watergate cover-up. However, there was never any evidence that he knew about, or was involved in, organizing the break-in. The House and Senate Republican leaders, Rep. John Rhodes of Arizona and Sen. Hugh Scott of Pennsylvania met with Nixon and urged him to resign. He did so two days later.

Even that abbreviated timeline shows the gravity of the situation and the lengths to which members of both parties in Congress went to investigate and establish the facts. That bipartisan process was about serious violations of the law, not policy differences between the president and members of Congress.

That is a crucial factor to keep in mind. Impeachment was not intended by the Framers to be used as a partisan political weapon or as a response by Congress to a president’s policies with which they disagree. That is what elections are for.

In the 50 years, we have learned that impeachment should not be used as a political weapon to resolve policy disputes, to retaliate for speeches or tweets by a president, or to short-circuit the results of an election with which the losing political party is unhappy.

Impeachment should only be used when a president is guilty of serious, substantial misconduct that betrays the public trust and renders him unfit to continue in office.