Bill de Blasio, the Democratic mayor of New York City, announced on Thursday that he was running for president, seeking to show that his brand of urban progressive leadership can be a model for the rest of the nation. It will be a steep challenge: He becomes the 23rd Democrat to enter the presidential race, and he does so against the counsel of many of his trusted advisers, and in the face of two centuries of history.

No sitting mayor has been elected to the presidency, and if Mr. de Blasio, 58, is to be the first, he must overcome daunting deficits in polls and fund-raising. His announcement, in a three-minute video titled “Working People First,” comes after months of groundwork that has included visits to early presidential primary states, a fund-raiser in Boston and a circuslike news conference this week in the lobby of Trump Tower.