Scott M. Stringer, the former New York City comptroller and 2021 mayoral candidate, said on Thursday that he would form an exploratory committee and begin raising funds for a possible primary challenge against Mayor Eric Adams next year.

The move caught much of the city’s Democratic establishment by surprise and signaled the start of a combative new phase of Mr. Adams’s mayoralty, as Mr. Stringer became the first Democrat to move toward directly contesting the mayor’s re-election.

Any primary challenge promises to be exceedingly difficult. No challenger has defeated an incumbent New York City mayor in a primary since David Dinkins beat Edward I. Koch in 1989.

But few of his predecessors have been held in such low regard in polls as Mr. Adams, who is confronting the city’s budget woes, an escalating migrant crisis and an F.B.I. investigation into his campaign. Other challengers may soon follow.

In an interview, Mr. Stringer offered a dire assessment of the nation’s largest city, saying that New Yorkers were facing a “crisis of confidence” in City Hall’s ability to manage the budget and the migrant crisis.

He cast himself as a seasoned alternative, ready to replace what he derided as the mayor’s “minimalist agenda” with his own policy plans to increase affordable housing construction and ease the impact of falling revenue in city coffers.