CLEVELAND — It was the biggest speech of Melania Trump’s life, and her husband, Donald, wanted it to be perfect.
The Trump campaign turned to two high-powered speechwriters, who had helped write signature political oratory like George W. Bush’s speech to the nation on Sept. 11, 2001, to introduce Ms. Trump, a Slovenian-born former model, to the nation on the opening night of the Republican National Convention.
It did not go as planned, and it has eclipsed much of the action at the party gathering in Cleveland, where delegates on Tuesday night formally nominated Mr. Trump for president.
The speechwriters, Matthew Scully and John McConnell, sent Ms. Trump a draft last month, eager for her approval.
Weeks went by. They heard nothing.
Inside Trump Tower, it turned out, Ms. Trump had decided she was uncomfortable with the text, and began tearing it apart, leaving a small fraction of the original.
Her quiet plan to wrest the speech away and make it her own set in motion the most embarrassing moment of the convention: word-for-word repetition of phrases and borrowed themes from Michelle Obama’s speech at the Democratic convention eight years ago.
The ridicule from both Democrats and Republicans was instant and relentless, disrupting what was meant to be a high point of the convention.
It was, by all accounts, an entirely preventable blunder, committed in front of an audience of 23 million television viewers, that exposed the weaknesses of an organization that has long spurned the safeguards of a modern presidential campaign, such as the free software that detects plagiarism.
“It just shouldn’t have happened,” said Matt Latimer, a White House speechwriter for President George W. Bush. “This was an easy home run speech: a successful, attractive immigrant talking about her husband.”
Nobody seemed more startled than Mr. and Ms. Trump, who arrived in New York on Tuesday morning after a flight from Cleveland to find themselves at the center of a bizarre uproar over authenticity, plagiarism and a knotty question: Why did the wife of the Republican nominee borrow passages from the wife of the current Democratic president?
Ms. Trump spent most of Tuesday out of sight, while her husband vented his frustration and anger throughout the day.
This account of how a speech written by professionals was transformed into the problematic version delivered on Monday night at the Quicken Loans Arena is based on interviews with more than a dozen people involved in and close to the Trump campaign. Many of them spoke on the condition of anonymity to disclose details that were supposed to remain confidential.
It reinforces dominant themes of Mr. Trump’s campaign that still linger from the primary, which his team has struggled to change: a deliberately bare-bones campaign structure, a slapdash style and a reliance on the instincts of the candidate over the judgments of experienced political experts, like Mr. Scully and Mr. McConnell..
The two original speechwriters were not aware of how significantly the speech had been changed until they saw Ms. Trump deliver it on television Monday night, along with the rest of the country.
In the prime-time address, Ms. Trump unfurled a sequence of life lessons — about how “your word is your bond,” about “your dreams and your willingness to work for them,” and the “integrity, passion and intelligence” of her parents — in the same sequence and using much of the same language that Mrs. Obama employed in 2008.
Just like Mrs. Obama, Ms. Trump explained how she wanted to pass those lessons on to her children and the children of the world. And just like Mrs. Obama, she offered a gauzy invocation about the limitlessness of aspirations when they are matched by determination.
In a series of evolving explanations, Trump aides and allies dismissed the episode as a trivial distraction, alternating between outright denial that Ms. Trump’s speech had used word-for-word phrases from Mrs. Obama and blaming the news media.
“Ninety-three percent of the speech is completely different,” declared Gov. Chris Christie of New Jersey. Paul Manafort, Mr. Trump’s campaign chairman, pegged the number of suspicious words at 50. “And that includes ‘ands’ and ‘thes’ and things like that,” he said on Tuesday.
Across the country, slack-jawed Republican political operatives and speechwriters expressed expletive-laden bewilderment at the organizational breakdown allowing such an episode to occur.
“It’s like some guy trying to paddle across a river in a rowboat who shoots a hole in his boat,” said Stuart Stevens, who wrote speeches for Mitt Romney and his wife, Ann, throughout the 2012 campaign. In interviews, alarmed Republican speechwriters outlined the layers of formal scrutiny, apparently disregarded by the Trump campaign, traditionally applied to almost every draft of a major convention address. They described word-by-word fact-checking by a dedicated team of experts and computer software designed to catch plagiarism. Several online programs, like DupliChecker, are available at no cost.
“It’s pretty standard,” Mr. Stevens said of the software, which detects overlap in word choice and sentence structure. “We used it.”
An urgent priority: avoiding the slightest hint of oratorical theft.
“The most cardinal rule of any speech-writing operation is that you cannot plagiarize,” said Mr. Latimer, the Bush speechwriter, who is now a partner at Javelin, a communications firm. If you do, he said, “you lose your job.”
That is unlikely to happen in the Trump campaign, which revolves around a freewheeling candidate with a fierce resistance to admitting error.
It was Jared Kushner, Mr. Trump’s son-in-law and top adviser, who commissioned the speech from Mr. Scully and Mr. McConnell — and praised their draft. But Ms. Trump decided to revise it, and at one point she turned to a trusted hand: Meredith McIver, a New York City-based former ballet dancer and English major who has worked on some of Mr. Trump’s books, including “Think Like a Billionaire.” It was not clear how much of a hand Ms. McIver had in the final product, and she did not respond to an email on Tuesday.
Research for the speech, it seems, drew them to the previous convention speeches delivered by candidates’ spouses.
The Trump campaign declined to say who or how many senior campaign officials read or reviewed the speech. But when Ms. Trump and her staff had finished revising the speech, virtually all that remained from the original was an introduction and a passage that included the phrase “a national campaign like no other.”
The controversy set off by the stumble spread rapidly from the political class to average Americans: African-Americans were angry that Ms. Trump had chosen to swipe the words of the country’s first African-American first lady, especially given Mr. Trump’s hostility to President Obama. Scores of Twitter users, deploying the hashtag #famousMelaniaTrumpQuotes, began to re-attribute famous lines, like the Rev. Dr. Martin Luther King Jr.’s “I have a dream,” to Ms. Trump.
But the mischievous teasing at times turned serious, as blacks invoked a painful history of prominent white figures stealing the work of black artists and presenting it as their own. “I’m not surprised Melanie plagiarized from Michelle,” wrote Yasmin Yonis. “White women have spent centuries stealing black women’s genius, labor, babies, bodies.”
To many Republicans, the lapse seemed frustratingly inevitable from a candidate who has not just eschewed the backstops of a major political campaign — he has mocked them as a waste of money. His campaign slogans, “America First” and “Make America Great Again,” echoed Pat Buchanan and Ronald Reagan. His social media graphics were crowdsourced on Twitter and Reddit by an aide who formerly managed Mr. Trump’s golf club in Westchester.
The mistakes have piled up. Last summer, Mr. Trump posted on Twitter his portrait superimposed over a picture of the White House and what turned out to be a stock image of Waffen-SS troops from World War II.
But this one stung, in part because everybody was watching.
Jon Favreau, a former chief speechwriter to President Obama, was home on his couch half-following Ms. Trump’s speech on TV while catching up on work Monday night. At first, he was skeptical of the criticism.
“Everyone says, ‘You work hard,’” Mr. Favreau said, reciting a line from the speech. “Political speeches are filled with clichés that are impossible to avoid.” But when he got to Ms. Trump saying, “Your word is your bond,” Mr. Favreau recalled, he stopped short.
“I remember Michelle saying, ‘Your word is your bond,’ and thinking I’ve never heard of someone saying that in politics,” Mr. Favreau said. “That was when I knew it might have been copied.”