Fareed Zakaria is the host of CNN's "Fareed Zakaria GPS." He is also a columnist for The Washington Post and the author of "In Defense of a Liberal Education" and "The Post-American World." The opinions expressed in this commentary are his own. View more opinion articles on CNN.

(CNN) Programming note: In a CNN Special Report, "Presidents Under Fire: The History of Impeachment," Sunday at 9 p.m. ET, host Fareed Zakaria examines the evolving role of presidential impeachment in US government.

Impeachment is a combustible issue, to say the least. As it is being talked about increasingly widely, I thought it might be time to revisit the three moments in American history when it ignited: during the presidencies of Andrew Johnson, Richard Nixon and Bill Clinton.

What can we learn from those tumultuous examples?

For my latest CNN special, I sat down with some experts to understand what impeachment is, how it works, and whether President Donald Trump could or should be impeached.

Impeachment is a complex issue, with political, legal and historical dimensions. It's one of the most basic democratic safeguards enshrined in the Constitution, yet few people really understand it.

The first important thing to realize is that impeachment is a political process. An impeachable offense is, at the end of the day, whatever Congress defines as such. But in a constitutional republic, shaped by law and history, what is the mandate given to Congress? What does the Constitution mean when it speaks of impeaching a president who has committed "high crimes and misdemeanors"?

In history's three main impeachment cases, Congress "never defined high crimes and misdemeanors," explained Timothy Naftali, a CNN presidential historian and a professor at NYU. Instead, Congress cited "abuse of power or a certain violation of the law -- perjury or obstruction of justice." Why? "Because the Founders never defined high crimes and misdemeanors."

Some background

There is some legal background that is relevant, or at least was relevant to the Founding Fathers when they wrote that phrase.

"That was language that they borrowed from the British," said Noah Feldman, a professor at Harvard Law School. "'High' means pertaining to high office.... So if your crime or misdemeanor has nothing to do with your office, you're not really covered by the Framers' idea of impeachment. But if it is connected, then your act is high."

Historically, that appears to have been a common feature. Members of Congress who have voted for impeachment "always come back to the idea that some action or some pattern of conduct by the chief executive represents a threat to our democracy and to our Constitution," according to Naftali.

The Founders' perspectives are valuable but they may have their limits. Most of them conceived of a presidency of extremely limited powers, and they would almost certainly be stunned at the size and scope of the White House's power today. In their view, Feldman notes, the excessive use of presidential power -- taking the country into war without explicit Congressional authorization -- would likely have been much clearer grounds for impeachment than lying about sex with an intern.

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Impeachment talk seems to have become more common in recent years. After Clinton, there have been cries to impeach George W. Bush, Barack Obama, and now Donald Trump. This is part of a worrying pattern.

"It really has been about 20 years -- which is to say from the Clinton impeachment -- that impeachment talk has so overtaken our political discourse," noted Josh Matz, co-author of "To End a Presidency: The Power of Impeachment."

Bush and Obama "ended their presidencies with about a third of the country supporting their impeachment," Matz added, but "President Trump came to office with about one third of the American public already supporting his impeachment. That's extraordinary."

"This is a power that's meant to deal with a president who's gotten out of control, not a president whose election you think was a terrible mistake. But there was this drastic and rapid shift from electoral loser to impeacher-in-waiting."

Impeachment and Trump

So has Donald Trump committed offenses that could be considered impeachable? Laundering money through real estate deals? Defrauding Trump University students? Misusing his foundation or evading taxes? Of course, such allegations have never been proven in a court of law.

Feldman told me, "Crimes that Trump may allegedly have committed before he had anything to do with the office of the presidency do not count as 'high crimes and misdemeanors,' and they would not be impeachable offenses in my view."

What about profiting off foreign governments while in office? Foreign countries have rented rooms and held events at Trump properties, perhaps to curry favor with the president. That could land Trump in trouble because the Constitution prohibits federal officeholders from receiving "any present [or] emolument... from any king, prince, or foreign state."

But Feldman advised against impeaching Trump over the Emoluments Clause.

"I don't really think that we'd want to impeach the president where the impeachment would depend on the technical meaning of a word -- 'emolument' -- that has never been considered by the courts and it was used by the Framers 200 and something years ago," he said.

Some people have accused Trump of obstructing justice by firing FBI Director James Comey. Could that lead to Trump's removal?

"Obstruction of justice is a charge that was used both against Richard Nixon and against Bill Clinton," Feldman pointed out. "And if it's real, it's a very strong ground for impeachment."

Regarding Trump's case, Feldman elaborated, "It's true that the director of the FBI works for the president and the president has the right to remove him on any whim that he might have, but the fact that the president can remove Comey doesn't mean that it's permissible for him to do it if he did it for gain."

To prove that, there would have to be a smoking gun, and "It's very hard to prove corrupt intent," Feldman said.

However, Feldman does see possible charges in another case, which he believes is "the closest parallel" to Nixon's case. (Recall that Nixon's scandal involved the break-in of the Democratic National Committee's headquarters by Republican operatives during the 1972 election.)

According to the sworn testimony of Trump's former lawyer Michael Cohen, Trump directed Cohen to make an illegal payment to porn star Stormy Daniels in order to influence the election.

"A president who distorts the electoral process and breaks the law in doing so is someone who is potentially impeachable," Feldman argued. "If there were evidence that Donald Trump further colluded with Russians in a way that undercut the legitimacy of the election, that would be an even deeper parallel to the Richard Nixon case."

These are, of course, the issues at the heart of Special Counsel Robert Mueller's enquiry and we will have to wait to see what evidence and conclusions his report contains.

Experts disagree about whether this or that charge is impeachable, but they do agree on one thing: the sword of impeachment must be wielded with great caution.

Naftali called it "almost an unusable option."

"Impeachment is capital punishment for a presidency, and impeachment represents an undoing of the democratic process," he said. "You are overturning an election.... It's something that Congress should not consider unless all other avenues are no longer open."

'A crisis of domestic governance'

What would an impeachment process look like in today's deeply divided America?

"A lot of folks think that impeachment is this mechanical process and that when the president commits high crimes and misdemeanors, [it] just falls out of the sky like some kind of sword of Damocles," Matz said. "I'm here to tell you that it doesn't. Congress has to decide whether impeachment is the right move."

Unless there is overwhelming proof accepted by the majority of the country, impeachment will not bring the country together.

"It creates a crisis of domestic governance," Matz warned. "It activates the worst kinds of partisan tribalism on all sides of the aisle. In the eyes of some, including some heavily armed parts of our population, it may seem like an illegitimate effort to depose somebody who finally can speak for them."

Feldman told me, "The only circumstances where I would actively support impeachment would be where there was evidence so glaring that failure to impeach would essentially show the hypocrisy of the whole system -- would show that we're willing to allow a president who has committed visible crimes in connection to his office to stay in office."

In other words, America might be too polarized today to deal with an impeachment honestly and responsibly. That's a dark verdict on the state of our politics, but it rings true. And it has a worrying consequence.

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"When you live in a world of broken politics and when you live in a world of extraordinary partisan polarization, it just may not be possible to generate the consensus necessary to use the impeachment power," Matz said. "That's a scary thought because the impeachment power does something important. There may be circumstances where we just can't wait for the next election, and I don't have a reassuring answer to that."

Matz invoked a famous line attributed to one of America's earliest sages: "Benjamin Franklin left the Constitutional Convention and he said, when asked about what plan of government they had created, 'A republic, if you can keep it.'"