DES MOINES — Mayor Bill de Blasio took the stage here on Tuesday evening, more than 1,000 miles from the city he governs, to outline his vision to revive a Democratic Party that he described as lost in recent years but on the precipice of a great comeback.

“We are at the beginning of a progressive era,” he predicted in a 35-minute speech before more than 150 liberal activists.

Mr. de Blasio was the headline speaker at the annual holiday party of Progress Iowa, a group that, like the mayor, hopes to pull Democratic politics to the left. In a dark suit and red tie, Mr. de Blasio hailed attendees as “an army of good.”

“We are not the party of elites,” he said. “We are not the party of big donors. We are the party of working people.”