Five years ago, Donald Trump bolted onto the political scene by falsely suggesting that President Obama wasn’t born in America.

Now, at the Democratic National Convention on Wednesday night, the president returned the favor — arguing that Trump’s values were fundamentally un-American, and that his candidacy was a unique threat to democracy in the United States.

In remarkably strong language, Obama suggested 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."

Apocalyptic rhetoric like this would have been unimaginable at the Democrats’ 2008 and 2012 conventions, against John McCain or Mitt Romney. And it would have been unimaginable against Jeb Bush or Marco Rubio or John Kasich, if one of them had won the nomination this year.

But Obama argued that this wasn’t about a choice between a Democrat and a Republican. Instead, he said he believes Trump threatens the very fabric of American society.

The president’s rhetoric about Trump was deeply unusual

Here were some phrases from Obama’s speech — a speech a sitting president made about a major party nominee to succeed him:

This is not your typical election. It’s not just a choice between parties or policies; the usual debates between left and right. This is a more fundamental choice – about who we are as a people, and whether we stay true to this great American experiment in self-government... ...What we heard in Cleveland last week wasn’t particularly Republican – and it sure wasn’t conservative. What we heard was a deeply pessimistic vision of a country where we turn against each other, and turn away from the rest of the world. There were no serious solutions to pressing problems – just the fanning of resentment, and blame, and anger, and hate. And that is not the America I know. ...I see Americans of every party, every background, every faith who believe that we are stronger together... And there is only one candidate in this race who believes in that future... ...I promise you, our strength, our greatness, does not depend on Donald Trump. In fact, it doesn’t depend on any one person. And that, in the end, may be the biggest difference in this election – the meaning of our democracy... ...anyone who threatens our values, whether fascists or communists or jihadists or homegrown demagogues, will always fail in the end.

Altogether, it’s a utterly scathing denunciation of the GOP nominee. Obama is arguing that while Trump may be American by birth, he is deeply, deeply un-American in his values.

The fact that Obama went this far shows how worried he is that Trump could win

Obama’s speech was upbeat and optimistic in tone on the surface. But the fact that he went so far in criticizing Trump shows how deeply worried he is that the billionaire might actually win.

Because while Obama has repeatedly expressed confidence in the wisdom of the American people, and faith that they’d eventually reject Trump, the truth is that, so far, they haven’t.

The president asserted, "We are not a fragile or frightful people. Our power doesn’t come from some self-declared savior promising that he alone can restore order."

"He’s just offering slogans, and he’s offering fear. He’s betting that if he scares enough people, he might score just enough votes to win this election," Obama said. "That is another bet that Donald Trump will lose. Because he’s selling the American people short."

Yet right now, Trump is neck and neck with Hillary Clinton in the polls, even after a convention that Obama said presented "a deeply pessimistic vision of a country where we turn against each other" and fanned "resentment, and blame, and anger, and hate."

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 should Trump win, and desperate to prevent that from happening.