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.

The video began with what has become the mayor’s tagline this year, as he has flirted with declaring his candidacy. “There’s plenty of money in this country, it’s just in the wrong hands,” he said, before highlighting his accomplishments as mayor and the ways that he has confronted Mr. Trump over immigration and climate change.

“I’m a New Yorker, I’ve known Trump’s a bully for a long time,” he said.

Mr. de Blasio then appeared on ABC’s “Good Morning America” with his wife, Chirlane McCray, seemingly intent on drawing the attention of Mr. Trump right out of the box, calling him “Con Don” — using Mr. Trump’s tactic of coining demeaning nicknames for rivals.

Asked what set him apart in the big Democratic field, Mr. de Blasio sounded what will likely be his central campaign theme: “Working Americans deserve better and I know we can do it because I’ve done it here in the largest, toughest city in this country.”

It did not take long for President Trump to respond on Twitter, calling Mr. de Blasio “the worst mayor” in the country.