In the summer of 2020, President Trump sent federal agents to Kansas City, Mo., as he blamed liberal mayors for a “shocking explosion” of “bloodshed.”

Mayor Quinton Lucas, a Democrat, bristled at the suggestion that local officials were to blame for his city’s spike in crime. And with distrust of law enforcement at a high after the police killing of George Floyd in Minneapolis that year, he worried about how federal law enforcement officers would conduct themselves on the ground.

Yet over the next few months, Mayor Lucas came to endorse parts of the federal mission, named Operation Legend after a 4-year-old Kansas City boy who had been killed by a wayward bullet as he slept.

Working with the local police, federal agents helped track down people with open felony warrants, recovered illegal guns and charged some suspects with federal crimes that can carry stiffer penalties than those available under state law. Now, as Mr. Trump renews his promise to tackle crime in American cities, Mayor Lucas says he could imagine welcoming the help — if it came with a clear strategy.

“I’d say yes in an instant,” he said, if federal agents were sent to help keep guns out of the hands of domestic abusers and teenagers, or to trace bullets used in shootings. “I’d go to the White House and have a press conference with them tomorrow.”