“That is the contribution of Black Lives Matter,” he added, “and it’s a reason, since learning about how that phrase was being used to push back on that activism, I’ve stopped using it in that context.”

The close examination of Mr. Buttigieg’s language and his handling of the police department is one of the first speed bumps for a campaign that has otherwise enjoyed a relatively smooth ride. Mr. Buttigieg announced this week that his presidential campaign had raised more than $7 million in the first quarter of 2019, a significant sum for a mayor who was little known outside of South Bend only a few months ago.

Nate Levin-Aspenson, a local organizer in South Bend, told CNBC that Mr. Buttigieg’s use of the phrase in 2015 “was not that long ago.”

“It was not a time when ‘all lives matter’ was a smart thing to say, or reflective of someone who is concerned about black people being killed by the police,” he said.

The Black Lives Matter movement began in 2013 in response to the acquittal of George Zimmerman, a neighborhood watch volunteer who fatally shot Trayvon Martin, an unarmed black teenager. The movement gained even greater recognition the following year after the police killings of Michael Brown and Eric Garner.

Soon, the phrase “all lives matter” also emerged. Some people who faced blowback for using the phrase said they had used it with good intentions as a way to bring people together. But others seemed to use it to rebut the idea that black people faced unique prejudice in the United States.

The fresh scrutiny of Mr. Buttigieg’s 2015 remarks has also resurfaced the allegations made by the police chief, Darryl Boykins, who sued the mayor in 2013. In federal court documents, Mr. Boykins — who could not immediately be reached on Thursday — said he had been illegally demoted from his job as chief in 2012 because he was black. Mr. Buttigieg has maintained the demotion was necessary because Mr. Boykins was the subject of a federal wiretapping investigation.

The case was settled in late 2013, court documents show. But a separate legal dispute continues between the City Council of South Bend and the group of police officers who were recorded over whether the tapes can be released. Mr. Buttigieg’s office has refused to release the tapes without a court order, and has said that doing so would put the administration in legal jeopardy.