Mr. de Blasio entered the presidential race late, and was the 23rd Democrat to declare. He faces challenges in everything from fund-raising to staffing, not to mention standing out in a crowded field.

Many New Yorkers don’t believe he should run, and many of Mr. de Blasio’s closest advisers warned him to stay out of the race. On Friday, he raced around Iowa with three staff members, all of whom were City Hall aides volunteering their time. Two residents of New York City public housing traveled to Sioux City to confront the mayor about funding for the troubled agency. Back at City Hall, dozens of student activists from the group Teens Take Charge held a rally to call for Mr. de Blasio to take more forceful action to integrate New York City's public schools, which are among the most racially segregated in the nation.

When asked how much money he had raised in the 24 hours since his announcement, the mayor declined to answer.

Mr. de Blasio will also be campaigning without the benefit of his two children, Chiara and Dante, who played important roles in his unexpected win in the 2013 mayoral race.

“Our children have their own lives. I can’t speak for them anymore,” said the mayor’s wife, Chirlane McCray, before Mr. de Blasio jumped in.

He said there was “no plan” for his children to take part in his presidential campaign in the “near term.” Mr. de Blasio will go straight from the campaign trail to Dante’s graduation from Yale University on Sunday.