“It’s never politically safe, or easy, to ask the most privileged Americans to pay more in taxes,” Mr. de Blasio said.

Still, some prominent Democrats say they are concerned that Mr. de Blasio, who has a short track record as mayor, is more focused on symbolism than substance.

The mayor “wants to be a big shot,” said William M. Daley, President Obama’s former chief of staff. If Mr. de Blasio wants a national profile, Mr. Daley said, he should “lay out specifics for urban America that are doable, not just, ‘We want the wealthy to be taxed more so we can spend their money somehow.’”

“Something specific that’s urban related,” Mr. Daley added. “Not just something amorphous, like ‘the progressive agenda.’”

Friends and colleagues of Mr. de Blasio’s say that his idea for a liberal coalition — it has not yet been given a formal name — is fueled by genuine moral dismay at the rise of economic inequality and that he wants to take advantage of his bully pulpit while it lasts.

“We’re talking about the state of the United States of America; we’re talking about the direction of the country at a time of crisis,” Mr. de Blasio told reporters in the Bronx on Tuesday, before his trip. “This is something that I’m going to speak about every chance I get, everywhere I go,” the mayor said.

Some of those opportunities are not exactly by chance. When Mr. Harkin, visiting Manhattan in September, suggested an Iowa visit to Mr. de Blasio, the mayor was intrigued, and he aggressively followed up. He enlisted one of his political consultants to work with contacts in Iowa to ensure that the trip would happen.