The possibility that President Trump will face impeachment no longer seems far-fetched. Congressional investigations — on the Trump organization, the Trump campaign, the Trump transition team and the Trump administration — are ramping up, while Special Counsel Robert Mueller seems ever closer to releasing his report on the depth of Russian interference in the 2016 election. Initial court filings and news reports suggest that the president may eventually be implicated in clear criminal activities, such as suborning perjury or obstructing justice. If so, the House will have to consider moving forward with impeachment proceedings against a president for the third time in our history.

Impeaching a president is always a political act. When scandal strikes the presidency, the decision to investigate, indict and remove a chief executive involves a great deal of political calculation on all sides. Partisan loyalties loom large, of course. Members of the opposition party typically make the case for impeachment; those in the president’s party rally to his defense until they no longer can’t.

Prosecutors can issue reports, journalists can uncover wrongdoings, and citizens can clamor for justice, but ultimately the responsibility for deciding about whether to take action lies with the House of Representatives. The standards for impeachment are surprisingly vague — the Constitution names “treason” and “bribery” as crimes for which federal officials can be removed from office, but it also features a catch-all category of “other high crimes and misdemeanors.”

Because the threshold for impeachment is ambiguous, different congressional majorities have offered wildly different rationales for when they would — or would not — impeach a president. What follows is a brief primer at the three cases of presidential impeachment that have rocked our democracy.

Andrew Johnson

Our country’s 17th president found himself in an awkward position from the start. A former Democratic senator from Tennessee, Andrew Johnson was the running mate for Republican President Abraham Lincoln on a “National Union” ticket in 1864. Despite his southern roots, Johnson denounced Confederates as “traitors” and argued “treason must be punished.”

Once he became president after Lincoln’s assassination on April 15th, 1865, Johnson dramatically changed course to the consternation of Republicans who championed a robust approach to Reconstruction. Johnson pushed for a light and lenient approach to former rebels, releasing Confederate leaders from government custody, appointing new governors to southern states and allowing their legislatures to pass ruthless new measures that curtailed the rights of freed slaves. In February 1866, he vetoed the Freedmen’s Bureau Bill. Belligerent, bigoted and often publicly drunk, Johnson railed against the “radical Republicans” who championed a strong Reconstruction of the South and feuded with Lincoln’s holdovers in his own Cabinet.

Troubled by what they saw in the White House, the Republican Congress sought to shield the administration officials who were acting as a check on Johnson’s growing insanities by passing the Tenure of Office Act of 1867, which required a president to seek the approval of the Senate not just for the appointment of high-ranking officials, as mandated by the Constitution, but for their removal as well. In February 1868, Johnson defied the law by firing Secretary of War Edwin Stanton. When Stanton refused to leave his office — he literally hunkered down in the War Department — Johnson’s enemies in Congress finally had grounds to initiate impeachment proceedings against him. Thaddeus Stevens, one of the champions of Radical Reconstruction, led the charge.

The House of Representatives voted to impeach Johnson almost immediately, in a 126-47 vote on February 24th, 1868 that broke down along party lines. Only after the vote did a House committee start drawing up the actual articles of impeachment, ultimately arraying eleven counts against the president, most of which centered on the Tenure of Office Act. The Senate trial began in March 1868, lasting for three long months. In the end, the vote to remove Johnson — taken shortly before the presidential election that would replace him — fell short by a single vote. Johnson finished his term and left office on March 4th, 1869.

Richard Nixon

The impeachment proceedings against Richard Nixon stemmed from his own political schemes.

During his first term, Nixon had angered many Democrats by aggressively flexing his presidential power and conducting secret military operations in Southeast Asia. Seeking to shore up his political standing in his 1972 re-election campaign, Nixon authorized the creation of a special unit in the White House to crack down on administration sources who were providing secret information to the press — a unit known as “the Plumbers,” because plumbers handle leaks. On June 17th, 1972, members of the unit were caught breaking into the headquarters of the Democratic National Committee at the Watergate office complex in Washington, D.C. The White House press secretary initially dismissed the break-in as little more than a “third-rate burglary” but it soon became clear that top officials in the Nixon White House were implicated in the crime and, soon after, the cover-up. Though Nixon defeated Senator George McGovern by a historic landslide, the scandal did not disappear.

Investigations proceeded along two tracks. In May 1973, Attorney General Elliott Richardson appointed Archibald Cox, a former solicitor general from the Kennedy administration, to serve as a special prosecutor into the Watergate break-in and its aftermath. When Cox insisted upon access to secret White House recordings, Nixon ordered him to be fired in an October 1973 confrontation known as “the Saturday Night Massacre.” Nixon hoped firing Cox would end the investigation, but public outrage prompted the appointment of a new special prosecutor, Leon Jaworski, who continued to press the case.

Meanwhile, the Democratic Congress conducted a major inquiry of its own. While the first congressional investigation into Watergate, chaired by Texas Representative Wright Patman in 1972, did not receive much political support, the next round of hearings proved to be a blockbuster. A Senate select committee, led by the conservative Democratic Sen. Sam Ervin of North Carolina, who liked to call himself an “ol’ country lawyer,” launched televised hearings in May 1973 that helped unearth evidence about the crime and the cover-up over the summer. “If the many allegations made to this date are true,” Ervin said when the hearings began, “then the burglars who broke into the headquarters of the Democratic National Committee at the Watergate were in effect breaking into the home of every citizen of the United States.” Later, in February 1974, the House Judiciary Committee launched a formal impeachment inquiry. The committee once again sought access to White House tapes, but the president refused, insisting that the doctrine of “executive privilege” protected the Oval Office conversations from ever becoming public.

The matter reached the Supreme Court in the landmark July 1974 case of US v. Nixon. The Court ordered the president to hand over the tapes in an 8-0 ruling. (Justice William Rehnquist, a former member of the Nixon Justice Department, recused himself from the decision.)

The tapes revealed in no uncertain terms that the president had been involved in the cover-up. The so-called “smoking gun” tape, in particular, provided clear evidence that the president had authorized a plan in which the CIA would be used to tell the FBI, falsely, that any investigation of the Watergate break-in would jeopardize national security. In late-July 1974, the House Judiciary Committee passed three articles of impeachment, for obstruction of justice, abuse of power and contempt of Congress. Republicans, after holding firm for months, finally abandoned the president in droves.

When it became clear to the administration that the full House would vote to impeach the president and the Senate would then move to remove him, Nixon realized he had no choice. He resigned from office on August 9th, 1974. “In the past few days,” he told the nation in a televised address, “it has become evident to make that I no longer have a strong enough political base in the Congress to justify continuing” his fight to stay in office.

Bill Clinton

The Clinton administration was beset by scandals from the start, as political opponents seized on any instance of wrongdoing. Republicans sought to delegitimize the president, while many Democrats were suspicious of him as well. Most of the investigations turned out to be nothing, despite the best efforts of Clinton’s political enemies to frame them as major scandals on par with Watergate, complete with their own “-gate” suffix: “Nannygate,” “Travelgate,” “Filegate,” etc. These inquiries ramped up significantly once Republicans took control of the House of Representatives after the 1994 midterms. (One inquiry into the possible misuse of the White House Christmas card list, for instance, involved 140 hours of sworn testimony before Congress.)

The two most prominent scandals stemmed from events that had transpired when Clinton was still Governor of Arkansas. The Whitewater investigation, named after a land deal in which the Clintons and some friends had invested, prompted the appointment in August 1994 of independent counsel Kenneth Starr, a former solicitor general in the George H.W. Bush administration. Despite spending four years and nearly $40 million investigating Clinton’s real estate dealings, Starr found no evidence of wrongdoing on his official mandate. Meanwhile, another high-profile scandal — rooted in allegations of sexual harassment brought by former Arkansas state employee Paula Jones — likewise turned up nothing. An U.S. District Court dismissed the case as a nuisance suit in April 1998.

While those investigations came up empty, they did unearth evidence of a different matter — a sexual affair Clinton had with White House intern Monica Lewinsky. Starr soon discovered that the president had lied about the affair, under oath, in a grand jury deposition he had given in the Jones case. Starr concluded that the president had committed perjury and released his report to the House of Representatives and also to the general public.

Believing the Starr Report contained sufficient evidence against the president, the House Judiciary Committee did not hold hearings. The topic of impeachment figured prominently in the 1998 midterm elections, which surprisingly resulted in a slight gain for Democrats in the House. During the lame duck session of Congress, House Republicans nevertheless passed two articles of impeachment in December 1998, on the grounds of perjury and obstruction of justice, in a largely party-line vote. The public reaction proved hostile, however, with Clinton’s approval ratings surging 10 points to an all-time high of 73 percent. The Senate, reading the public mood, acquitted the president on both articles.

Kevin M. Kruse and Julian E. Zelizer are historians at Princeton University and the co-authors of a new book, Fault Lines: A History of the U.S. Since 1974 (Norton)