In the more than 230 years of US history, only three presidents have suffered the humiliation of an impeachment trial.
Yet in the last half-century, this dramatic procedure seems to have become "normalized" in a way analysts say is unlikely to be reversed soon.
Impeachment is a rarely used and powerful weapon, provided by the US Constitution as a means to remove a president found guilty of "treason, bribery, or other high crimes and misdemeanors."
The Constitution sets a high bar for the removal of a president requiring the votes of 67 of the 100 senators, and Trump, like his two predecessors, was expected to be acquitted Wednesday in his trial before the Senate.
Johnson's impeachment came nearly 100 years after the nation's founding. But it has been just over 21 years since Clinton's impeachment trial.
And only 24 years before that, Richard Nixon resigned his presidency to avoid the indignity of impeachment over the Watergate dirty-tricks scandal.
More recently, the threat of impeachment was raised, unsuccessfully, against George W. Bush over the invasion of Iraq, and even against Barack Obama on rather shakier grounds, such as the false claim that he was born outside the United States.
'An alarming trend'
"Trump's impeachment is less a historic moment than a continuation of an alarming trend," Michael Gordon, a former member of the Clinton administration, wrote on the Business Insider website.
"Considering the increasing division in our country...," he added, "it's likely that impeachment will soon become normalized."
Trump's lawyers warned of that during the Senate trial.
Without dwelling on the charges facing the Republican president - that he withheld aid from Ukraine to pressure it to investigate his political rivals - his lawyers asserted that even if the facts were proven, they would not justify Trump's removal from office.
"The bar for impeachment cannot be set this low," argued Jay Sekulow, Trump's lead outside counsel in the trial.
That, he said, would "impact the functioning of our constitutional republic and the framework of that Constitution for generations."
Members of a minority party, added Republican Senator Rand Paul, could use impeachment "as a weapon to get rid of a president they simply don't like."
A divided America
But James Thurber, a political science professor at American University in Washington, declared he was skeptical of this argument.
"There is no doubt that America has been polarized, is polarized and will continue to be polarized, because of a variety of phenomena, not the least of which is that we have this real urban/rural split," he told AFP.
But the impeachment procedure is "such a serious thing that people will not want to go through that again," he affirmed. "There will be other mechanisms to compete between parties."
The weapon of impeachment can, of course, be turned against those who wield it. Republicans paid a price at the ballot box the November after the Senate acquitted Clinton of charges stemming from his liaison with White House intern Monica Lewinski.
In Trump's case, the polls are mixed. It won't be clear until the November 3 elections how big a factor - for or against - impeachment might be in voters' minds.
'Be more careful'
For Thurber, the future of impeachment will depend largely on the attitude of the next few presidents, who he said may be impelled by the current impeachment drama to "be more careful on how they speak and how they act than this president."
Another political scientist, Mark Rozell of George Mason University near Washington, uttered that "if the impeachment bar has now been lowered, it may not be an altogether bad thing"; it might give future presidents added incentive to "operate within the legitimate boundaries of their constitutional power."
"They should be looking over their shoulder and wondering every single day, 'How would this look on the front page of the newspaper tomorrow? How would Congress react to this? Would this withstand a challenge in the judicial branch?'"
If Trump is re-elected, he seems unlikely to change his own approach to governance, Thurber said.
Indeed, if Democrats retain control of the House of Representatives in the fall, "I could see the House going forward with another impeachment."
by Charlotte Plantive, Agence France-Presse