Under practically any other circumstances, what Barack Obama just did would strike many people as entirely inappropriate.

The sitting president denounced one of the two major candidates to succeed him in the office as "unfit to serve" and suggested that he wouldn’t be able to "function as president."

Obama suggested that, unlike past presidential nominees of both parties, Donald Trump is uniquely lacking in "basic knowledge," "basic decency," "judgment," and "temperament." He implied Trump wouldn’t respect the United States’ "norms and rules," its "constitutional traditions," or the "rule of law."

In his Democratic convention speech last week, the president went even further, suggesting Trump was a "homegrown demagogue" who "threatens our values" much like "fascists," "communists," and "jihadists" do, and who could imperil "this great American experiment in self-government."

It’s hard to imagine any recent past president intervening in an election like this. Imagine George W. Bush saying this about Obama. Imagine Bill Clinton saying this about Bush. Imagine Obama saying it about Jeb Bush, Scott Walker, or Marco Rubio.

But then again, Trump is no normal candidate. And it’s a sign of how badly this election has gone off the rails that Obama’s statements seem positively mild compared to everything Donald Trump has said during his campaign.

An abridged version: Trump has called for banning people of the Muslim faith from entering the country, derided Mexican immigrants as criminals and "rapists," taken a stance on NATO that could actually make a nuclear war more likely, said a judge born in Indiana should be disqualified from a case because of his "Mexican heritage," suggested his primary rival’s father was involved in President Kennedy’s assassination, generally showed a weakness for inane conspiracy theories, suggested that Obama sympathizes with terrorists, tried to smear the family of Captain Humayun Khan for criticizing him, and preemptively claimed that this fall’s election will likely be "rigged."

Oh, and have we forgotten how Trump first burst onto the political scene this decade — by questioning whether President Obama was really born in America?

With all this in mind, Obama has clearly decided that things aren’t normal, and he shouldn’t pretend they’re normal. He is mounting an unprecedented response to a presidential candidacy he views as an unprecedented threat to the country.

Trump is the opposite of Obama in both style and substance

In many respects, Trump’s candidacy seems like it was engineered in a lab to terrify Barack Obama.

Obama rose to national prominence by arguing that there’s "not a black America and white America and Latino America and Asian America, there’s the United States of America," and has been consistently optimistic about racial progress. But Trump has exploited white Americans’ racial resentments, demonizing Hispanic immigrants and Muslims.

Obama has deep reserves of self-control and a famously "cool" temperament. But Trump seems to lack any self-restraint whatsoever, and repeatedly flies off the handle, viciously attacking anyone who defies him, whether they’re a news anchor or a Gold Star mother.

Obama has long been optimistic — overly optimistic, in the eyes of many — that America and the world’s problems can be solved through wonky, logical, technocratic, rational means. But Donald Trump is ignorant and lazy, constantly bungling facts and misspeaking about policy issues he won’t bother to put in the time to understand.

Having spoken so often about how difficult a job the presidency is — "the only things that land on my desk are tough decisions," he’s said — Obama has a special perspective on the job’s duties and difficulties. So when he says Trump is "unfit to serve as president" and that he simply couldn’t "function" in the job, he doesn’t say it lightly.

Obama must also be terrified that Trump could win

In his DNC speech last week, Obama also expressed confidence in the wisdom of the American people, and faith that they’d eventually reject Trump.

"We are not a fragile or frightful people," the president asserted. "Our power doesn’t come from some self-declared savior promising that he alone can restore order.… He’s betting that if he scares enough people, he might score just enough votes to win this election." He added: "That is another bet that Donald Trump will lose. Because he’s selling the American people short."

But the truth is that, so far, the public hasn’t overwhelmingly rejected Trump. Hillary Clinton seems to be leading the polls after getting a bounce from the convention, but right now it doesn’t look like a Democratic landslide is in the offing. Instead, around 45 percent of the American electorate, if not more, looks likely to back the GOP nominee.

Indeed, the fact that President Obama went so far in criticizing Trump shows how deeply worried he is that Trump might actually win.

As Matt Yglesias writes, Obama knows his own legacy is at risk in this election. But he seems concerned about more than that. If we take his rhetoric seriously, he’s deeply concerned about the future of the country and American democracy under President Donald Trump, and desperate to prevent that from happening.