WASHINGTON - No American president had been impeached since Andrew Johnson a century earlier when the House launched formal impeachment proceedings against Richard Nixon in the fall of 1973.
But once the door to impeachment was flung back open, it would reopen again and again.
Three presidents - Nixon, Bill Clinton and, now, Donald Trump - have faced impeachment inquiries in just the past four decades.
Nixon resigned in 1974 to avoid almost certain impeachment. Clinton earned a dubious place in history in 1998 by becoming only the second president to be impeached. All signs point to Trump soon becoming the third impeached president following House Speaker Nancy Pelosi's announcement on Thursday that Democrats would proceed with articles of impeachment.
Impeachment, an extraordinary constitutional punishment used only once against an errant president during the first two centuries of the republic, has evolved into a more habitual part of contemporary political discourse.
The answer, scholars say, lies in the no-holds-barred nature of modern-day partisan warfare, the idiosyncrasies of American political campaigns and the desire to rein in presidents as the executive branch's powers have expanded.
"Impeachment, although it is a constitutional process, is evidence in some ways that the system has failed and we need to resort to extraordinary remedies," said Ross Baker, a political scientist at Rutgers University-New Brunswick.
"Impeachment is the fire extinguisher on the wall," Baker said. "But it's better not to have the fire."
In Trump's case, House Democratic leaders argue that while they recognize impeachment is extraordinary, his actions - urging a foreign government to investigate a political rival - were so egregious that he left them no choice but to pursue an impeachment investigation. Trump accuses Democrats of pursuing a partisan witch hunt through the impeachment inquiry.
'A decent set of chaps'
When the Constitution was written, some of the framers of the document resisted including an impeachment clause. "There was a thought you didn't need impeachment because you had elections," said Frank Bowman, a law professor at the University of Missouri and the author of a book on presidential impeachment.
But delegates to the constitutional convention in Philadelphia adopted the impeachment clause after much debate because they feared that someday it might be necessary to remove a president from office.
It wasn't until Johnson nearly a hundred years later that presidential impeachment would be put to the test.
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The unpopular 17th president was impeached by the House in 1868 after a bruising fight with Congress over his post-Civil War Reconstruction policies and his removal of Secretary of War Edwin Stanton. Thirty-five senators voted to find him guilty of the charges - a single vote short of the two-thirds majority required for conviction and removal from office.
No other president would face possible impeachment until Nixon got entangled in the Watergate scandal. One reason, Bowman said, is that all of the presidents between Johnson and Nixon "were a pretty decent set of chaps."
Some may have been mediocre, and others may have downright lousy at their jobs. But, "by and large, the presidents were a pretty good lot and didn't do the kind of stuff the framers would think of as being impeachable," Bowman said.
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Nixon represented a new era that would continue with Clinton and Trump.
The scandal that led to Nixon's resignation began with a 1972 break-in of the Democratic National Committee headquarters at the Watergate hotel by five men who were later revealed to have ties to the president's reelection campaign.
An extramarital affair and charges that he lied under oath would trigger the case against Clinton, who was impeached by the House but acquitted by the Senate.
The impeachment investigation into Trump was sparked by charges that he pressured Ukraine to announce an investigation into a political rival, former Vice President Joe Biden, and Biden's son Hunter, who sat on the board of a Ukrainian energy company.
"We happen to be in a period in which the people who secure the presidential nominations in elections are more problematic," Baker said.
He blames some of that on the nation's personality-driven presidential campaigns.
"When you are looking at potential presidents, what we know about them mostly is what they tell us about themselves," Baker said. "Even though journalists and scholars try to make some appraisals of them, self-presentation turns out to be an incredibly important thing, and some people are just preternaturally gifted at presenting themselves without blemishes.
"Even those who have multiple blemishes, like Donald Trump, basically have a good line of self-promotion."
University of North Carolina law professor Michael Gerhardt, who has written books on impeachment, offers another theory. Though impeachment is still rare, it's happening to more presidents because more people are watching their every move, he said.
"The presidency itself is covered more closely (by the press), scrutinized more closely and held potentially more accountable," said Gerhardt, who testified before the House Judiciary Committee on Wednesday that Trump's actions were worse than the misconduct of any prior president.
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Perhaps more than anything, Bowman said, the rise of hyperpartisanship has fueled the recent push for presidential impeachments.
When Nixon faced impeachment, the Democratic and Republican parties each included liberals and conservatives among their ranks. Nixon, in fact, had been counting on the support of Southern Democrats to keep him in office but was forced to face reality - and, ultimately, resignation - as their support crumbled.
Political parties in the post-Nixon era are characterized more by ideological purity than diversity, which breeds fierce partisan warfare in which the goal is to take down the other party's leader, Bowman said.
"Success or failure becomes defined as the rise or fall of your ideological faction led by the president," he said.
Expansion of presidential powers
Over the last century, multiple presidents have expanded the president's powers - from Teddy Roosevelt's reliance on executive orders to carry out much of his progressive agenda, Woodrow Wilson's heavy involvement in international affairs during World War I, and Franklin Roosevelt's New Deal reforms that gave rise to a large bureaucracy headed by the president.
Bowman and others suggest there may be a correlation between the increase in presidential powers and the renewed interest of impeachment as a way to keep the president in check.
The tussle between the White House and Congress over presidential powers escalated when Nixon defied a congressional subpoena to turn over White House tapes, transcripts and other documents during the Watergate hearings.
The matter would eventually end up in the courts, and Nixon would lose. In a decision that limited a president's powers to claim executive privilege, the Supreme Court ruled on July 24, 1974, that Nixon had to turn over the requested documents.
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Another landmark ruling came during Clinton's presidency when the Supreme Court decided on May 27, 1997, that a sitting president cannot be immune from civil lawsuits filed against him or her for acts committed before taking office. The decision cleared the way for Paula Jones, who claimed Clinton had propositioned her, to proceed with a lawsuit against Clinton while he was still president.
"For some conservatives, they have wanted to push back on these judicial decisions which they have thought cut back on the president's power," Gerhardt said.
Hence, Trump has refused to hand over records to House Democrats and has tried to block top administration officials from testifying before House committees leading the impeachment inquiry against him.
In partisan warfare, impeachment can be a way to expose alleged misbehavior or wrongdoing even when there's little chance that it will end with a president's removal from office, Bowman said.
"That's what we're going to see here (with Trump)," he said. "Unless a miracle occurs, we're not going to remove Trump as a result of this process. Impeach him? Yes. Remove him? No."
Michael Collins covers the White House. Reach him @mcollinsNEWS.
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This article originally appeared on USA TODAY: Nixon, Clinton, Trump: Why impeachment is more common