One of the long-running story lines of the Republican presidential primary season was this: What would Donald Trump do if ever his beloved polls turned against him? If he was no longer winning, how would he act? Whom would he blame?

Lucky for Trump, there was almost never a moment during his run to the Republican nomination in which he trailed, so those questions never really got answered. There was one instance, however, when Ben Carson briefly passed Trump in Iowa over last fall, and we got a glimpse of an embattled Trump who wasn't terribly appealing — as a candidate or a person.

In a rambling 95-minute speech in Fort Dodge, Iowa, in November, Trump lashed out at, well, everyone. "How stupid are the people of Iowa? How stupid are the people of the country to believe this crap," he asked the crowd in reference to Carson's story about an apparent stabbing incident in his younger years. Trump referred to rival Carly Fiorina as "Carly whatever-the-hell-her-name-is." He famously/infamously declared, "I know more about ISIS [the Islamic State militant group] than the generals do." And on and on.

Well, that Trump — angry, petulant, petty — returned Saturday night in a campaign appearance in Manheim, Pa.

Like in November in Iowa, Trump found himself backed into a corner — a poor showing in the first presidential debate was made worse by his inexplicable, multi-day attacks on former Miss Universe Alicia Machado and punctuated by a devastating New York Times story about his taxes.

And, like in November, Trump came unglued.

The Post's Jenna Johnson was there, and her report on the Trump speech is, frankly, stunning stuff. I could excerpt the whole story, but here's just one example of Trump's tirade:

It took Trump nearly 25 minutes to read the brief statement because he kept going off on one angry tangent after another — ignoring his teleprompters and accusing Clinton of not being “loyal” to her husband, imitating her buckling at a memorial service last month, suggesting that she is “crazy” and saying she should be in prison. He urged his mostly white crowd of supporters to go to polling places in "certain areas" on Election Day to "watch" the voters there. He also repeatedly complained about having a "bum mic" at the first presidential debate and wondered if he should have done another season of “The Apprentice.”

This video, of Trump imitating Clinton's near-collapse at a Sept. 11 memorial service last month, has to be seen to be believed.

The Trump in that video is the exact opposite of presidential. The word that kept coming to my mind when I watched it was "nasty." He seems mean, angry, vindictive. None of those words tend to be what people use to describe presidents.

Simply put: If you had questions before Saturday night about whether Trump had the proper temperament to hold the job he is seeking, it's hard to imagine that you don't have serious doubts today.

True character tends to be revealed when times are tough. Anyone can be magnanimous, happy and generous after a win. It's a hell of a lot harder to maintain that dignity and charitableness after a defeat.

Trump has shown throughout this campaign that he runs well while ahead. His chiding of his opponents, his dismissiveness of the political press — it all plays great when he is on top of the political world.

But, last night in Manheim, he showed what we got glimpses of almost a year ago in Iowa: When he's down, Trump is like a cornered animal. He lashes out — at everyone. That is when he's at his most dangerous — to his own prospects and those of the party he is leading.

What Donald Trump is doing on the campaign trail

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MANCHESTER, NH - NOVEMBER 7: Republican presidential candidate Donald Trump speaks during a campaign event at SNHU Arena in Manchester, NH on Monday November 07, 2016. (Jabin Botsford/The Washington Post)