In what passes for normal inside Donald Trump’s unorthodox campaign for president, he flew from Arkansas to Iowa on his Trump-emblazoned jet on Friday, arrived the next morning at a candidate forum without any prepared remarks and, wearing a bright red tie that evoked his days on "The Apprentice," told the world exactly what he thought about Sen. John McCain’s reputation as a war hero.

It was an improvised fit of pique, roundly and vigorously denounced by his rivals all weekend, that exposed the biggest vulnerability of Trump’s campaign for president: It is built entirely around the instincts and grievances of its unpredictable candidate – and does not rely on a conventional political operation that protects presidential hopefuls from themselves.

In a reaction that highlighted the problem, Trump on Sunday refused to apologize for declaring that McCain is "not a war hero" because he was captured and instead boasted in an interview that his talk in Iowa had aroused "the biggest standing ovation" of the day.

The remarks about McCain, the 2008 Republican presidential nominee, ended any qualms party officials had about criticizing Trump for fear of alienating his supporters and might normally have led to days of backpedaling and extended explanations. Even as Trump insisted that no one was troubled by his comments, his small group of aides emailed one another about how to respond to the growing criticism.

But the word "sorry" is not in Trump’s lexicon, and apologizing was not an option that was discussed, people privy to the internal debate said.

In a sign of the seat-of-the-pants nature of his campaign, it sent out a series of dissonant messages, some trying to tamp down the controversy (by showing support from veterans) and others going on the attack (especially of the media).

It remains to be seen whether Trump’s standing in public opinion surveys will suffer from the episode with McCain. But recent national and early-primary state polls put Trump in the top tier of candidates.

If nothing else, the weekend reaffirmed that Trump is running a presidential campaign on his own unique terms.

Never mind that his top rivals for the Republican nomination treat campaigning like a full-time job. For Trump, the task of seeking the White House occupies half his time, he estimated in an interview. ("It’s probably 50-50," he said.)

The rest of the Republican field’s top tier has cast a wide net to find experienced political aides. But Trump has plucked much of his team from inside his own corporate empire. (The risumi of his Iowa co-chairwoman: She was a contestant on "The Apprentice.")

While his competitors may be busy working through thick stacks of books on world affairs to prove their qualifications, Trump says he has little use for such. ("One of the problems with foreign policy," he explained, "is that it changes on a daily basis." As a busy man, he added, he prefers newspapers.)

There is no real policy shop churning out position papers, or for that matter a well-staffed central headquarters plotting his long-term message, or speechwriters drafting – or modulating – his words. And there is a circular, interoffice quality to what the campaign does with its money.

But the dangers of Trump’s approach are now being laid bare. Bare-bones improvisation, which seemed sufficient to fuel his ascent in the polls, is starting to backfire.

Trump faces a moment of real reckoning. Is the man known for the catchphrase "You’re fired!" willing to soften his caustic language? Will he slog through the grueling rituals of a long campaign? And, above all, will his message keep resonating – or will his own outlandishness undermine his candidacy, turning it into his latest exercise in brand-building?

So far, tellingly, he is continuing to criticize McCain, and has shown little interest in building a conventional campaign.

"I’m not trying to arm the country," he said in a lengthy interview last week, emphasizing that he does have staffs in the first three states in the nominating process: Iowa, New Hampshire and South Carolina.

The reality is that Trump is pulling off something that, for now, requires little planning, spending or organization: He is giving voice to a profound rage in the Republican electorate – over economic displacement, illegal immigration and America’s diminished place in the world.

"I have a pulse to the ground," he added. "I think I know what’s wrong with the country, and I think I’ve been able to portray that in a way that people agree with."

Unlike in the past when he mused about the presidency, Trump is putting a team on the ground as well. He has hired at least four workers in New Hampshire, for example, and has visited Iowa nearly a dozen times. He hired a campaign manager, Corey Lewandowski, a New Hampshire resident who previously worked for Americans for Prosperity, a political action committee financed by the conservative billionaire Koch brothers.

Overall, he has spent $1.4 million on his campaign, most of it out of his own pocket, and less than half of what Jeb Bush and Marco Rubio have spent.

In an interview on Sunday, Trump showed no sign of changing his style.

He checked off the names of those attacking him over his remarks about McCain, insulting Louisiana Gov. Bobby Jindal and former Texas Gov. Rick Perry in the process.

"Jindal, who has nothing," he said. "Rick Perry – I mean Rick Perry, give me a break here."

© 2015 The New York Times Company