Only two US presidents - Andrew Johnson and Bill Clinton - have been formally impeached by Congress and no US president has ever been removed from office through impeachment.
Although many other presidents have been threatened with impeachment by political foes without gaining any real traction in Congress, only two other US presidents other than Johnson and Clinton have faced formal impeachment inquiries in the House of Representatives - Richard Nixon and Donald Trump.
The framers of the Constitution intentionally made it difficult for Congress to remove a sitting president. The impeachment process starts in the House of Representatives with a formal impeachment inquiry and if the House Judiciary Committee finds sufficient grounds, its members write and pass articles of impeachment, which then go to the full House for a vote.
A simple majority in the House is all that's needed to formally impeach a president. But that doesn't mean he or she is out of a job. The final stage is the Senate impeachment trial. Only if two-thirds of the Senate find the president guilty of the crimes laid out in the articles of impeachment is the POTUS removed from office.
Although Congress has impeached and removed eight federal officials—all federal judges—no president has ever been found guilty during a Senate impeachment trial. Andrew Johnson came awfully close, though; he barely escaped a guilty verdict by one vote.
Bill Clinton: Impeached in 1998
Clinton was plagued by legal troubles and scandals from the moment he entered the White House. In 1993, Clinton and his First Lady, Hillary, were the subject of a Justice Department investigation into the so-called Whitewater controversy, a botched business deal from their days in Arkansas. And in 1994, Clinton was sued for sexual harassment by Paula Jones, who claimed Clinton exposed himself to her in a hotel room in 1991.
Interestingly, it was a combination of both legal cases that would ultimately lead to Clinton's impeachment. Independent counsel Kenneth Starr was appointed by the Justice Department to investigate the Whitewater affair, but he couldn't find any impeachable evidence. Meanwhile, lawyers for Jones got a tip that Clinton had an affair with a 21-year-old White House intern named Monica Lewinsky, a claim that both Lewinsky and Clinton denied under oath.
Starr switched the focus of his investigation when he received 20 hours of taped phone conversations between Lewinsky and Linda Tripp, a former White House colleague, in which Lewinsky alludes to the affair. Starr then got the FBI to fit Tripp with a wire to meet with Lewinsky at a Ritz-Carlton hotel outside Washington, DC, when Lewinsky again admitted to a sexual relationship with the president.
When the story went public, Clinton was forced to address the accusations on national television.
"I want you to listen to me," Clinton famously said. "I did not have sexual relations with that woman, Miss Lewinsky. I never told anybody to lie, not a single time, never."
Starr's investigative team ended up producing a long and lurid report detailing Clinton's sexual dalliances with Lewinsky and providing evidence that Clinton lied under oath (perjury) in an effort to obstruct the Starr investigation.
On December 19, 1998, the House of Representatives voted along party lines to impeach Clinton on two separate counts: perjury and obstruction of justice. But in the ensuing five-week Senate trial, Clinton was acquitted on both counts.
Despite a very public and embarrassing scandal, and being only the second president in history to be impeached, Clinton's job approval rating peaked at 73 per cent in 1999.
Andrew Johnson: Impeached in 1868
Johnson was elected as Abraham Lincoln's vice president in 1864. The toughest decision facing Lincoln's second term was how to reestablish ties with the Confederate states now that the Civil War was over. Lincoln's plan for Reconstruction favoured leniency while so-called "Radical Republicans" in his party wanted to punish Southern politicians and extend full civil rights to freed slaves.
Lincoln was assassinated only 42 days into his second term, leaving Johnson in charge of Reconstruction. He immediately clashed with the Radical Republicans in Congress, calling for pardons for Confederate leaders and vetoing political rights for freedmen. In 1867, Congress retaliated by passing the Tenure of Office Act, which barred the president from replacing members of his cabinet without Senate approval.
Believing the law to be unconstitutional, Johnson went ahead and fired his Secretary of War, an ally of the Radical Republicans in Congress. Johnson's political enemies responded by drafting and passing 11 articles of impeachment in the House.
"Sir, the bloody and untilled fields of the ten unreconstructed States, the unsheeted ghosts of the two thousand murdered negroes in Texas, cry [...] for the punishment of Andrew Johnson," wrote the abolitionist Republican Representative William D. Kelley from Pennsylvania.
Johnson was impeached in the House of Representatives by 126 votes to 47 but narrowly avoided a two-thirds guilty verdict in the Senate by a single vote. After his acquittal, he served out the rest of his term and became the first (and only) former US president to be elected to the Senate.
Richard Nixon: Resigned in 1974
Despite being complicit in one of the greatest political scandals in US presidential history, Richard Nixon was never impeached. He resigned before the House of Representatives had a chance to impeach him. If he hadn't quit, Nixon would likely have been the first president ever impeached and removed from office, given the crimes he committed to cover up his involvement in the Watergate break-ins.
On July 27, 1974, after seven months of deliberations, the House Judiciary Committee approved the first of five proposed articles of impeachment against Nixon, charging the president with obstruction of justice in an effort to shield himself from the ongoing Watergate investigation. Only a handful of Republicans in the judiciary committee voted to approve the articles of impeachment, and it was unclear at the time if there would be enough votes in the full House to formally impeach the president.
But everything changed on August 5, 1974, when the Supreme Court ordered Nixon to release unedited tapes of his Oval Office conversations with White House staffers during the Watergate investigation. The so-called "smoking gun" tapes included Nixon proposing the use of the CIA to obstruct the FBI investigation, and paying hush money to the convicted Watergate burglars. The transcript included the following:
NIXON: How much money do you need?
JOHN W. DEAN: I would say these people are going to cost, uh, a million dollars over the next, uh, two years. (Pause)
NIXON: We could get that.
Once the tapes were made public, Nixon got word from Republican congressional leadership that all but 15 Senators would likely vote against him in an impeachment trial, more than enough to remove him from office. To save himself the indignity of becoming the first sitting president fired by Congress, Nixon resigned on August 8, 1974.
Nixon was pardoned of criminal charges by Gerald Ford, but many of his Watergate conspirators weren't so lucky. Most of his White House legal counsel, including John Dean, went to jail for their involvement in Watergate.