It’s Day 1 of a New Mayor’s Race in New York

One Democratic primary candidate, State Senator Zellnor Myrie, cut a video offering himself as an antidote to the perceived political chaos. Others warned that Mr. Adams was undermining his party’s chances in November. And a centrist lawyer who is running as an independent, Jim Walden, quickly challenged the mayor to a debate.

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

Some seasoned political analysts confidently predicted his switch would have little effect on the final result in a city where Democrats outnumber Republicans six to one, and where Mr. Adams’s job approval ratings have fallen to record lows.

“He wasn’t going to be re-elected yesterday, and he isn’t going to be today,” said Howard Wolfson, a Democratic strategist and former deputy mayor who helped Michael R. Bloomberg win re-election in 2009, when he ran on the Independence and Republican Party lines. “His campaign is irrelevant.”

But Mr. Adams’s decision raises at least the possibility that the city could be headed toward a freewheeling, multicandidate general election unlike any it has seen since 1977. After losing the Democratic primary to Edward I. Koch that year, Mario M. Cuomo — Andrew Cuomo’s father — ran in the general election on the Liberal Party line, only to fall short again.

In this case, it is becoming easier to imagine a scenario in which as many as a half dozen candidates are still competing in November. They could include Mr. Adams; Andrew Cuomo, who leads the Democratic primary field; Curtis Sliwa, the leading Republican candidate; and Mr. Walden.

The left-leaning Working Families Party, a noted enemy of Mr. Cuomo, could also choose to give its general election ballot line to one of the race’s liberals and Mr. Cuomo’s rivals.

Keith L.T. Wright, chair of the Manhattan Democratic Party, said that he would be supporting the Democratic nominee in the general election but that it was still too early to write Mr. Adams off.

“There is still a base of folks who are loyal to him and, in a general election, who the hell knows?” he said. “Unusual circumstances deserve unusual strategies. It’s been done before under different circumstances.”

Ana María Archila, co-director of the New York Working Families Party, said the party was prepared to run a candidate in the general election, depending on the outcome of the Democratic primary.

The group has already endorsed four candidates, including Zohran Mamdani, an assemblyman from Queens who is backed by the Democratic Socialists of America; Brad Lander, the city comptroller; Adrienne Adams, the City Council speaker; and Mr. Myrie, a state senator from Brooklyn.

“If one of our people comes really close in the Democratic primary, then that probably becomes the natural path,” Ms. Archila said.

If Mr. Cuomo ultimately wins the primary, she added, “that will be a huge factor for us” in deciding to run an opposing candidate in November.

Amid the tumult, Mr. Adams faced questions for the first time as a candidate at a forum hosted on Thursday afternoon by the Rev. Al Sharpton, previewing his strategy to win back the support of New Yorkers.

Greeted with warm applause, Mr. Adams struck a defiant posture. He suggested that the bribery and fraud charges against him were racially motivated, criticized old friends who “broke my heart” by abandoning him during the legal saga, and argued that the case for his re-election was obvious based on his record.

“I wanted to run in a Democratic primary, but I have to be realistic. I have to let New Yorkers know what I have done,” he said, adding that the case “has overshadowed our success.”

The mayor mostly eschewed attacking Mr. Cuomo or other potential rivals, though he brushed aside a suggestion that his run could allow a Republican to win City Hall.

“Listen, I think that if you had 20 people in the race, it would never help Curtis Sliwa be mayor,” he said.

While other Democrats at the forum, including Ms. Adams, attacked the mayor, Mr. Cuomo took a gentle approach during his time onstage. He said that he did not want to “second-guess” Mr. Adams’s decisions as mayor but implied that he, Mr. Cuomo, was prepared to take a tougher stance against Mr. Trump.

“I don’t think you can fight him hard enough,” he said.

Mr. Sliwa, on the other hand, was openly rooting for the mayor to run as an independent. In an interview, he said that Democrats were “at each other’s throats” and predicted they could split their share of the vote.

“Then it’s like the Final Four in basketball,” said Mr. Sliwa, who won nearly 30 percent of the vote citywide against Mr. Adams in 2021. “It’s a jump ball.”

In the near term, the impact on the June Democratic primary appeared more straightforward. Even before his decision on Thursday, Mr. Adams had been badly trailing in the polls and was not expected to win.

Allies of Mr. Cuomo insist that he is best positioned to consolidate what remains of Mr. Adams’s working-class Black and Latino base. But the mayor’s exit from the primary contest could also clear the lane for solitary attacks on Mr. Cuomo and offer fellow Democrats a fresh opening to get the attention of voters.

Mr. Adams, for his part, expressed newfound optimism about his own candidacy on Thursday, after he won a long-sought reprieve in his legal case.

Mr. Adams could further scramble the race, though, if he continues to veer rightward. That might alienate his traditional supporters, but could theoretically allow him to compete for independent and Republican voters, particularly if Mr. Trump lent his support.

The two men have courted one another for months. After the judge’s ruling dismissing the case against Mr. Adams on Wednesday, the mayor held up a copy of a book attacking the federal “deep state” written by Kash Patel, Mr. Trump’s F.B.I. director, and urged New Yorkers to read it.

Mr. Adams has several more weeks to collect the petitions needed to qualify for the ballot as an independent.

But his rebooted campaign, which is strapped for staff and money, got off to a rocky start. Mr. Adams did not show up to a live TV interview Thursday morning, after his team had confirmed the appearance.