WASHINGTON – Richard Nixon nearly got impeached. Bill Clinton did. And Donald Trump could be next.

A common thread running through all three imperiled presidencies?

Hillary Clinton.

In the 243-year history of the republic, just four U.S. presidents have faced impeachment inquiries. In a bizarre plot twist that reads like bad fiction, Clinton – a former first lady, senator, secretary of State and almost-president – is connected to three of them.

“You cannot make my life up. Really,” Clinton joked recently with television host Stephen Colbert about her strange ties to the impeachment cases against the 37th, 42nd and 45th presidents.

Clinton was still known as Hillary Rodham when, fresh out of Yale Law School, she was hired by the House Judiciary Committee to work on the impeachment case against Nixon during the Watergate scandal. The memo they produced is an official government document and is still relevant today in large part because it helps define what is considered an impeachable offense.

Two decades later, Clinton was America’s first lady, literally standing by her man, President Bill Clinton, in the White House Rose Garden, nodding her head in agreement as he decried “the poisonous venom of excessive partisanship” following his impeachment in 1998.

Two decades after that, Hillary Clinton would run for president herself and win the popular vote but lose the Electoral College – and the presidency – to Trump, who is now facing an impeachment inquiry amid accusations that he pressured a foreign power to investigate a political adversary, former Vice President Joe Biden.

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Clinton’s office declined to comment for this article.

But Frank Bowman, a law professor at the University of Missouri and the author of a book on presidential impeachment, described Clinton’s connection to impeachment proceedings against three presidents as “an odd coincidence” and “a testament to Hillary Clinton’s longevity as part of the national political life.”

“She’s a part of (the impeachment story),” Bowman said, “but that’s because she has been a part of the national story for so long.”

'Impeachment ... is a very big thing'

Before Watergate in the early 1970s – a scandal that would eventually end Nixon's presidency – no American president had faced impeachment since Andrew Johnson a century earlier.

Amid Watergate, with impeachment no longer a constitutional relic but a real possibility, the House Judiciary Committee tapped Clinton and a bipartisan team of young staffers to research the history of impeachment and explore possible grounds for impeaching Nixon.

“The staff and members of the House understood that considering the impeachment of a president is a very big thing, and they wanted to make sure they understood the history and the law,” said Michael Gerhardt, a University of North Carolina law professor and expert on impeachment.

Clinton and others on the bipartisan team produced a 64-page memo detailing the origins of impeachment dating to the British Parliament more than 400 years ago and the constitutional grounds that would be needed to impeach an American president.

Among their conclusions: Criminal conduct is not needed to satisfy the definition of “high crimes and misdemeanors,” one of the thresholds the U.S. Constitution spells out for impeachment of a public officeholder.

The 1974 memo “is a tremendous piece of work,” Bowman said. “I don’t know how much contribution Secretary Clinton had to that work, but given its length, I think it really covers the ground nicely. And I very much agree with almost all of its conclusions, particularly its conclusion about the kinds of things that are impeachable.”

The memo, an official government document, served as something akin to a road map for lawmakers as they embarked on the impeachment of Nixon. Over the course of four days in late July 1974, the House Judiciary Committee debated five articles of impeachment against Nixon and eventually approved three, accusing him of obstruction of justice, abuse of power and contempt of Congress.

A week later, Nixon resigned before the full House voted on the articles of impeachment.

Impeachment history:The times an American president was impeached (and the one time it came close)

Forgotten until Monica Lewinsky

The memo that Clinton and her colleagues wrote was largely forgotten until 1998, when Bill Clinton’s presidency was put in jeopardy by his extramarital affair with White House intern Monica Lewinsky.

House Republicans dusted off and pored over the document as they began to contemplate articles of impeachment against the husband of an author of the memo.

“People paid a lot of attention to it – partly because it was substantive and partly because of the irony,” said Gerhardt, who himself cited the memo when he testified before a House committee contemplating Bill Clinton’s impeachment.

Then-Rep. Bob Barr, a Georgia Republican who was one of the most vocal proponents of impeaching Bill Clinton, even used the memo to taunt the president’s wife.

In a Wall Street Journal op-ed addressed to “Dear Mrs. Clinton,” Barr wrote that when he first raised the notion of impeaching her husband, “little did I realize your scholarly work 23 years ago would provide clear historical and legal basis and precedent for my proposition.”

“I presume – but I must ask whether – you stand by your research and analysis today,” Barr wrote before concluding with a caustic zinger: “Thank you, Mrs. Clinton, for giving Congress a road map for beginning our inquiry.”

On Dec. 19, 1998, Bill Clinton became the second American president to be impeached when the House, voting mostly along party lines, approved two articles of impeachment accusing him of perjury to a grand jury and obstruction of justice. The Senate acquitted him of both charges the following January.

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Hillary Clinton would go on to be elected U.S. senator from New York, serve as secretary of State under Barack Obama and lose her own bid for the presidency to Trump, who is now the subject of an impeachment inquiry led by House Democrats.

And once again, Clinton figures into the impeachment discussions.

'Break glass for emergency' moment

Drawing on her own experience, Clinton has said in recent interviews that House Democrats had no choice but to open an impeachment inquiry against Trump following a whistleblower’s claims that Trump pressured Ukraine President Volodymyr Zelensky to investigate Biden and Biden’s son, Hunter Biden, who served on the board of a Ukrainian energy company.

Clinton “has incredible insight to the impeachment process both personally and professionally,” said author Amie Parnes, who has written a pair of books on Clinton’s two unsuccessful campaigns for president.

“She’s seen what goes into an (impeachment) inquiry and also on the flip side, in her husband’s case, how it can weigh on the president facing impeachment and, even in her case, how it impacts the president’s family,” Parnes said. “And I think that’s why you’ve seen her really wrestle with the implications of impeachment even against someone like her former opponent President Trump."

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A couple of months ago, Parnes said, Clinton talked about how impeachment shouldn’t be used for “trivial political” purposes.

“But I think, as with many Democrats, her views solidified with the whistleblower on the Ukraine call,” Parnes said. “I think for her this was a 'break glass for emergency' moment, even if some say she’s almost too close to it.”

Clinton’s impeachment memo has held up well over the years and could again serve as an important resource for lawmakers as they contemplate whether to file formal impeachment charges against Trump, Gerhardt said.

“I don’t think they are going to have to go back and reinvent the wheel,” he said. “This (memo) is sort of a good foundation, and my guess is the staffers have read it and it will be helpful to the current members.”

The prospect that Clinton’s memo could help bring down the man who dashed her own presidential ambitions is “the ultimate irony for Secretary Clinton,” Parnes said, “and in some ways, the ultimate vindication for her.”

“If she only knew then what she knows now."

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