For nearly two weeks, Mayor Eric Adams has insisted that he would not resign over the investigations swirling around his administration, and he stuck to that message on Wednesday night as news of his indictment on federal charges became public.

But at least one person has been readying himself for a different outcome: Jumaane Williams, who, as New York City’s public advocate, would become mayor if Mr. Adams stepped down.

Mr. Williams has been slowly working behind the scenes to prepare for the job, and began talking with his allies about the scenario as early as last year, not long after the mayor’s cellphone and other electronic devices were seized in November, according to two people familiar with the matter.

More discussions have taken place this month in the wake of the abrupt resignation of Lisa Zornberg, the mayor’s chief legal counsel. Mr. Williams has also spoken with his staff, including the public advocate’s counsel, about how a transition might proceed, according to a person familiar with the matter.

Mr. Williams told reporters recently that he doubted the mayor’s ability to navigate the federal investigations, and that he was ready to take over if necessary.

“That’s part of the job,” he said. “Is it something that you want to happen? No.”

As news of the mayor’s indictment circulated, the list of Democrats calling for his resignation grew to include at least three of Mr. Adams’s primary challengers and the Working Families Party, a prominent progressive group.