No one would ever have called this year’s race for mayor of New York City a foregone conclusion — not with a disgraced former governor, a surging democratic socialist and an incumbent battling corruption charges all in the running.

But on Thursday, it tipped into entirely uncharted territory.

The primary cause was Mayor Eric Adams, who, newly liberated from his criminal case, made a surprise 6 a.m. announcement that he would forgo the Democratic primary and run as a political independent this fall.

The decision was a pragmatic next step for a politician who has flamboyantly courted President Trump and his MAGA base, infuriating his own party as he sought a way out of his legal jeopardy. And it came just hours before candidates were required to deliver thousands of petitions to qualify for the June primary ballot.

But Mr. Adams’s opponents wasted little time trying to capitalize. A group whose catchy D.R.E.A.M. campaign — “Don’t Rank Eric or Andrew for Mayor” — had sought to persuade voters to leave Mr. Adams and former Gov. Andrew M. Cuomo off ranked-choice ballots quickly changed its name to reflect its now-singular mission: “Don’t Rank Evil Andrew for Mayor.”

One Democratic primary candidate, State Senator Zellnor Myrie, cut a video offering himself as an antidote to the perceived political chaos. And a centrist lawyer who is running as an independent, Jim Walden, quickly challenged the mayor to debate.

New York City has a long history of candidates switching parties or running on third-party ballot lines — sometimes successfully. Rarely, though, has a candidate done it seemingly so haphazardly as Mr. Adams, a onetime Republican who as recently as Wednesday insisted he still planned to run as a Democrat.