At the core of disagreements about the Trump-Russia affair’s place in U.S. history is a definitional question: What is a scandal anyway?

Scandals are big news, of course, but many had very little direct impact on the world beyond the corruption of some government officials. Failed policies, on the other hand, can directly wreak havoc on millions of people. That doesn’t mean scandals are unimportant. Even mild corruption can be harmful to democracy, and full-on assaults on the rule of law threaten the entire possibility of self-government.

All administrations suffer policy failures of varying degrees. Few are guilty of the kinds of criminal, ethical or constitutional violations that major scandals are made of.

While there’s no single best metric for assessing the size of a presidential scandal, we know the key factors. One is indictments and convictions. Another is the rank of those guilty and the proximity of the president to the crimes and abuses of power. Most importantly: How threatened was the rule of law?

By any reasonable measure, the Trump-Russia scandal already ranks as the biggest since at least 1986, when Iran-Contra was ravaging the Reagan administration. Nothing else needs to emerge and none of the unproven allegations need to be confirmed, either. What we know about this scandal so far is:

• Russia attempted to influence the 2016 election.

• The Trump campaign, including the candidate himself, openly welcomed those efforts.

• It also welcomed them covertly in private communications that have since emerged.

• The campaign and early White House had several characters who were compromised by their foreign involvements.

• Various Trump administration folks have been caught lying about contacts with Russia or Russians, and the president attempted to derail the investigation in several ways, including firing the FBI director.

• As NBC’s First Read details, the Mueller investigation has already produced indictments on 19 people, including Trump’s first national security adviser (with a guilty plea) and a campaign chairman.

That’s an enormous scandal. There’s nothing even remotely comparable between Iran-Contra and the Trump administration. Let’s rewind, administration by administration.

Barack Obama: Benghazi, IRS

The Obama administration was essentially free of major scandals. Benghazi? It was a policy disaster, no doubt, in terms of the events leading to the attack itself, which left four Americans dead. One can certainly argue that Obama’s Libya policy was a total failure as well. As a scandal, however, there’s almost nothing there. Several congressional investigations (as well as an internal State Department investigation) found no wrongdoing even close to the scale of the Russia story.

The other Obama scandals were similarly minor, regardless of the accusations thrown around by Republican-aligned media. The IRS “scandal” consisted of misguided attempts to enforce a foolish law, but it eventually was revealed that they did it without regard to ideology and partisanship. The Hillary Clinton server scandal did, apparently, uncover wrongdoing, but a thorough investigation yielded nothing more than a scolding. The Clinton uranium “scandal” didn’t even rise to that level.

George W. Bush: Iraq and Plame

The biggest scandal in the George W. Bush administration involved the overselling of the Iraq war. The Valerie Plame affair was a significant event, which eventually yielded the indictment and conviction of the vice president’s chief of staff. More broadly, there are still fair questions about the extent to which the intelligence on Iraq was “cooked” and how much it mattered. Still, the core problem with Iraq was poor policy choices by an administration operating within the lines, not the way that policy was sold. And a Senate panel’s investigation on prewar intelligence, which lasted five years, did not uncover illegal activity.

Bill Clinton: Monica Lewinsky

The Monica Lewinsky scandal? President Bill Clinton was certainly guilty of having an affair and lying about it. He was impeached and, eventually, given relatively minor punishments, including suspension of his law license. Republicans alleged a more serious conspiracy involving large-scale obstruction of justice, but not only did that argument go nowhere during the Senate trial, independent counsel Ken Starr never brought indictments against the others supposedly involved in the scheme. The Clinton scandal was a serious one, but in the end it was purely about the president’s personal behavior, which makes it hard to rank very highly.

George H.W. Bush

The George H.W. Bush administration was notably scandal-free, and just four years long.

Ronald Reagan: Iran-Contra

This was serious indeed. Contrary to his own policy, President Ronald Reagan approved arms sales to Iran in exchange for help freeing U.S. hostages held in Lebanon.

That would have a political bombshell when it was revealed, but the even bigger story was that the administration funneled the proceeds from the arm sales to U.S.-backed rebels in Nicaragua (the “Contras”) after Congress had cut off funding to those rebels. That’s a big deal: Presidents and the executive branch cannot just spend money wherever they want. Even worse, there were apparently plans to regularize the maneuver, setting up a regular funding flow for covert activities beyond the reach of Congress. That’s a serious subversion of the rule of law.

There also was a cover-up, but Reagan pushed for full investigation and disclosure. Eventually, several high-ranking officials were indicted, including a secretary of defense and two national security advisers. Some were convicted, some with convictions overturned on appeal, and eventually George H.W. Bush pardoned most of those involved.

Is Trump-Russia more serious (from what we know now) than Iran-Contra? It’s a judgment call. It’s very possible that Iran-Contra was more of a threat to the rule of law than Trump-Russia. On the other hand, Reagan’s involvement in Iran-Contra was minimal, and his biggest mistake was indifference.

Richard Nixon: Watergate

There are plenty of myths about Watergate — that it was only a minor crime, and that Richard Nixon’s real mistake was the cover-up. Not so. The break-in at the Democratic National Committee (at the Watergate complex) was only one of a series of crimes and abuses of presidential power committed by the White House staff and the Nixon re-election campaign. The president himself instigated these violations, even if he wasn’t always fully informed of operational details. As Carl Bernstein and Bob Woodward said on the 40th anniversary of Nixon’s resignation, “Watergate was a brazen and daring assault, led by Nixon himself, against the heart of American democracy: the Constitution, our system of free elections, the rule of law.”

Just the list of high officials indicted and convicted suggests the magnitude of the scandal, beyond the obvious one of Nixon’s resignation ahead of certain impeachment and conviction, and the likely prosecution of the former president derailed by Gerald Ford’s pardon for Nixon. Among those convicted:

• Nixon’s chief of staff

• His domestic policy adviser

• The political adviser

• The White House counsel

• Not one, but two attorneys general, one of whom was also the campaign chairman; and

• The deputy campaign director.

Most of them (almost certainly including Nixon) were guilty of crimes committed by June 1972, when the second Watergate break-in went wrong and the cover-up began. The cover-up itself was far more organized and extensive than anything revealed about Trump obstruction of justice so far.

Some claim that Trump-Russia is bigger than Watergate, but I think that’s a very hard case to make on the evidence so far, and even reasonable conjecture based on what’s been supported doesn’t really get us there, in my view. But for a scandal to reach the level of Iran-Contra and perhaps exceed it is a very big deal, even if it doesn’t approach Watergate. And we still don’t know what else Robert Mueller knows.

Jonathan Bernstein is a Bloomberg View columnist. Follow him on Twitter: @jbview

To send a letter to the editor about this article, submit online or check out our guidelines for how to submit by email or mail.