Not again. A day after Donald Trump’s wife, Melania, was found to be plagiarising chunks from a Michelle Obama speech, his son, Donald Jr, was accused of parroting phrases lifted from previously published work.

In what had seemed a smooth, confident and well-received speech, Trump Jr had described the malaise in America. “Our schools used to be an elevator to the middle class,” he complained. “Now they’re stalled on the ground floor. They’re like Soviet-era department stores that are run for the benefit of the clerks and not the customers.”

As in the case of Melania on Monday, the Twitterati soon pounced. Comedy Central’s The Daily Show highlighted an article by Frank H Buckley published two months ago by American Conservative magazine. “What should be an elevator to the upper class is stalled on the ground floor,” it read. “Our schools and universities are like old Soviet department stores whose mission was to serve the interests of the sales clerks and not the customers.”

It was all the more extraordinary because, after the Melania debacle, it seemed fair to assume that Trump would hire an infinite number of monkeys to check his son’s speech and ensure it contained not a syllable of copying. Buckley himself, however, did not seem upset. “I was a speechwriter for this speech,” he told Business Insider. “So I’m afraid there’s no issue here.”

Indeed, but for the previous farce, a speechwriter’s self-plagiarism would probably have passed without incident. But in the current climate, to use one old phrase may be regarded as a misfortune; to use two looks like carelessness.

The pity of it was that, like Melania, Trump Jr had in fact been a more agreeable speaker than many of the professional politicians the GOP convention has witnessed, many of them full of bile and rancour. With his tanned complexion and slicked-back hair, the 38-year-old looks like an actor playing the US president in a TV drama set in Washington. And he was perhaps the ultimate Trump spokesman in that he was selling the tycoon’s peculiar version of the American dream.

How can a billionaire businessman from New York be the one who “gets” the struggling working class? It is the paradox that has been at the heart of the Trump phenomenon. Describing himself as “the son of a great man”, Trump Jr attempted to square the circle. Regular guys, common sense and capitalism: good; intellectuals, experts and fancy universities: bad.

Watched by thousands of delegates, some waving “Make America great again” banners, Trump Jr recalled that, from as soon as he could walk, he was at his father’s side on the frontline. “He didn’t hide out behind some desk in an executive suite,” he said. “He spent his career with regular Americans. He hung out with the guys on construction sites, pouring concrete and hanging sheetrock. He listened to them and he valued their opinion as much, and often more, than the guys from Harvard and Wharton locked away in offices away from the real world.

“He’s recognised the talent and the drive that all Americans have. He’s promoted people based on their character, their street smarts and their work ethic, not simply paper credentials. To this day many of the top executives in our company are individuals that started out in positions that were blue collar, but he saw something in them and he pushed them to succeed.”

Trump Jr continued: “His true gift as a leader is that he sees the potential in people that they don’t even see in themselves. The potential that other executives overlook because their résumés don’t include the names of famous colleges and degrees. I know he values those workers and those qualities in people because those were the individuals he had my siblings and me work under when we started. That he would trust his own children’s formative years to these men and women says all you need to know about Donald Trump.

“We didn’t learn from MBAs. We learned from people who had doctorates in common sense ... It’s why we’re the only children of billionaires as comfortable in a D10 Caterpillar as we are in our own cars. My father knew that those were the guys and gals who would teach us the dignity of hard work from a very young age. He knows that at the heart of the American dream is the idea that whoever we are, wherever we’re from, we can get ahead, where everyone can prosper again.”

And the central argument was that, according to the great man theory of history, Donald Trump can reanimate that dream because we are all workers on his construction site.

His son said: “I know when people tell him it can’t be done, that guarantees that he gets it done. I know that when someone tells him that something is impossible, that’s what triggers him into action. When people told him it was impossible for a boy from Queens to go to Manhattan and take on developers in the big city, rather than give up, he changed the skyline of New York.”

In this most unorthodox of election years, there was something quite traditional about Trump Jr’s drawing of battle lines: his father the superhero of capitalism and competition; the Democrats representing big state regulation and stultification. “It’s called the free market. And it’s what the other party fears. They want to run everything, top-down, from Washington. The other party gave us a regulatory state on steroids.”

As for Trump’s rival: “If Hillary Clinton were elected, she’d be the first president who couldn’t pass the basic background check. It’s incredible. Hillary Clinton is a risk that Americans can’t afford to take.”

Earlier, Trump’s daughter Tiffany, 22, had taken the stage, disarmingly admitting that she was nervous and giving her father a PR boost. “I never expected to be here, addressing the nation,” she said. “I’ve given a few speeches in classrooms, but never in an arena with 11 million people watching.”

She told the crowd that she kept her report cards going back to kindergarten, not just for her grades but the notes her father scribbled on each one. “For me, the measure of a parent is how much they bolster you when you’re down,” she said, telling the audience that her father had always supported her. She told how Trump comforted her after the death of a close friend. “Without his support, I don’t know how I would have made it through.”

Her friends find Trump to be “so friendly, considerate, funny, and real”, she said.

There have been no claims of plagiarism against Tiffany so far.

