CLEVELAND | Donald Trump’s campaign on Tuesday tried to brush off charges of plagiarism and recover quickly from an unforced stumble as he advanced within steps of officially securing the Republican nomination for president.

Conversation on Day 2 of the party’s national convention was dominated by talk of the previous night’s speech by Trump’s wife, Melania Trump, which included lines at matched Michelle Obama’s 2008 convention address.

At a time when the campaign had planned on casting its one-man political tornado as a reliable leader capable of running a winning campaign, it was instead dogged by worries about its operations and facing pressure to hold someone accountable.

Trump’s campaign and allies described the controversy as a meaningless distraction, but offered no explanation for why two passages in the speeches — each 30 words or longer — matched nearly word-for-word.

“This is totally blown out of proportion,” Trump adviser Paul Manafort told The Associated Press. “They’re not even sentences. They’re literally phrases. I was impressed somebody did their homework to think that that could be possibly done.”

The controversy over what was otherwise a well-received testimonial by Mrs. Trump to her husband’s character came at an ill-timed moment. Barring a last-minute jolt to the proceedings, the New York businessman was hours away from a roll call vote to make official something the political establishment once deemed impossible.

“His quest will finally come to an end,” Manafort said. “It will have happened.”

The campaign had hoped to relish the moment, having quieted persistent concerns about its competence and pushed past raucous party divisions. The plagiarism question ensures neither worry will be far behind.

On Monday, the Cleveland arena resembled the convention-floor battles of old as aggrieved anti-Trump Republicans protested the adoption of rules aimed at quashing an already flailing effort to deny him the prize.

The roll call vote, set for Tuesday, is typically a quaint bit of political theater, heavy with ceremonial flourishes, good cheer and puffery about the virtues of each state.

This time, dissident delegates from several states planned to insist on abstaining or backing other candidates, said Regina Thomson of Colorado, a leader of a group calling itself Free the Delegates.

On Monday, the floor flight gave way to a lineup of hard-edged speeches in which Republicans painted a grim picture of the country’s future and an evener darker view of Democrat Hillary Clinton.

Speakers accused Clinton of lying, “putting all of our children’s lives at risk,” and threatening the nation’s security. When delegates chanted “Lock her up!” retired Gen. Michael Flynn responded from the podium: “You’re damn right. There’s nothing wrong with that.”

With a rock-star entrance, Trump changed the tone and introduced his wife, Melania. She traced her own life story, coming to America from Slovenia, and painted a softer, more rounded portrait of her husband than the hard-nosed, insult-throwing candidate voters have seen at every turn.

“If you want someone to fight for you and your country, I can assure you, he is the guy,” Mrs. Trump told delegates.

Her speech contained several striking resemblances to Michele Obama’s eight years ago. For example, when she said her parents taught her “that you work hard for what you want in life, that your word is your bond and you do what you say and keep your promise, that you treat people with respect.”

Obama, in her 2008 speech, said she was raised to believe that “you work hard for what you want in life, that your word is your bond, that you do what you say you’re going to do, that you treat people with dignity and respect.”

Other sections of Mrs. Trump’s Trump speech also tracked closely with Obama’s remarks.

Republican National Committee chairman Reince Priebus said Tuesday he would “probably” fire his speechwriters if they lifted passages from someone else’s remarks. Former Trump campaign manager Corey Lewandowski said he agreed.

“When you do something that is so egregious that the story today is not how great Melania Trump was and how successful this convention is going to be, but it’s because a staff person didn’t do their job properly, there absolutely has to be accountability,” he said on CNN. “I know accountability in the Trump campaign. I know what it’s like to be fired from the Trump campaign.”

The campaign said Monday night’s speech was written by a “team of writers” who had spoken with Mrs. Trump about her life experiences and included fragments that reflected her thinking.

Mrs. Trump’s speech was part of a four-day campaign to show the softer side of the tough-talking showman. On Tuesday night, two of his children will pick up where she left off. Tiffany Trump, his 22-year-old daughter from his marriage to Marla Maples, and Donald Trump Jr., his eldest son and an executive vice president at The Trump Organization, will take the podium.

Former GOP presidents, the home-state governor and other top Republicans have skipped Trump’s convention, but Tuesday’s bill highlights some establishment support.

House Speaker Paul Ryan, Senate Majority Leader Mitch McConnell, New Jersey Gov. Chris Christie planned to address the convention along with Sen. Steve Daines, who had earlier planned to go fly-fishing in Montana.