"This is entirely behind me," Weiner said at news conference, just hours after he confirmed exchanging a newly disclosed round of sexually explicit photos and text messages with a woman online.
At the news conference, he acknowledged some of the activity took place after he resigned from the House two years ago for the same sort of behavior.
The story broke earlier in the day when the gossip website The Dirty posted the X-rated correspondence and quoted the woman, who was not identified.
Weiner turned the microphone over to his wife, Huma Abedin, who reaffirmed her support for her husband and said the matter is "between us."
"I love him. I have forgiven him. And as we have said from the beginning, we are moving forward," said Abedin, a longtime adviser to former Secretary of State Hillary Rodham Clinton.
The allegation could severely test voters' willingness to forgive Weiner, who has said he spent the two years since the scandal trying to make things right with his wife and earn redemption. Three of his rivals for mayor immediately called on Weiner to drop out of the race.
The 48-year-old Democrat, who resigned his House seat in June 2011 after acknowledging having sexual conversations with at least a half-dozen women, has been near the top of most mayoral polls since his late entry into the race this spring.
"I said that other texts and photos were likely to come out and today they have," Weiner said in a statement issued by his campaign earlier in the day. "I want to again say that I am very sorry to anyone who was on the receiving end of these messages and the disruption this has caused."
The woman with whom he exchanged the messages told The Dirty that she was 22 when she began chatting with Weiner on the social networking site Formspring. She said their online relationship began in July 2012 and lasted for six months.
She claimed Weiner used the alias "Carlos Danger" for their exchanges, but she knew she was talking to the former congressman.
The exchanges posted on The Dirty consist of sexually explicit fantasizing about various sex acts. At one point, the man reported to be Weiner wrote, "I'm deeply flawed."
The woman said Weiner promised to help her get a job at the political website Politico and suggested meeting in a Chicago condo for a tryst.
The woman claimed that she and Weiner exchanged nude photos of themselves and engaged in frequent phone sex. The Dirty ran a pixelated photo of what appears to be a man's genitals.
"This was a bad situation for me because I really admired him. Even post scandal, I thought he was misunderstood. Until I got to know him. I thought I loved him. Pretty pathetic," the woman was quoted as telling the website.
She said he later asked her to destroy the evidence of their chats. She insisted that she never had sex with Weiner or received any payment from him.
The woman claimed her relationship with Weiner "fizzled" in November 2012. She said she last heard from him this past April, when his intention to run for mayor was first revealed in a New York Times Magazine profile.
His wife, who was pregnant when the sexting scandal broke in 2011 and gave birth months later, has played a large and increasing role in his mayoral campaign. She made an appearance in his campaign kick-off video, has led his fundraising effort and recently made her debut on the campaign trail. Two weekends ago, she walked hand-in-hand with Weiner as they talked to voters on a Harlem street.
Two of his mayoral rivals -- Public Advocate Bill de Blasio and former City Councilman Sal Albanese, both Democrats, and billionaire John Catsimatidis, a Republican -- quickly called on Weiner to abandon his quest for office, as did a lesser candidate in the race.
"Enough is enough," said de Blasio. "The sideshows of this election have gotten in the way of the debate we should be having about the future of this city."
Another mayoral hopeful, city Comptroller John Liu, stopped short of calling for Weiner to bow out, but suggested his "propensity for pornographic selfies is a valid issue for voters."
The other leading Democratic candidates, including City Council Speaker Christine Quinn and former City Comptroller Bill Thompson, did not immediately comment on the new revelations.
The disclosure suddenly puts Weiner's indiscretions, judgment and candor back in the forefront of his campaign and could test voters' confidence in him, political analysts said.
Some voters have said they felt Weiner had atoned for his past and were willing to give him a second chance. But a third, after hearing allegations that his misbehavior continued after his resignation?
"It makes it tougher to believe this is behind him," said Democratic former state Assemblyman Michael Benjamin, now a political consultant.
But given the corps of voters who have shown willingness to forgive Weiner's prior behavior, the latest revelation may not be a campaign knockout, said Jerry Skurnik, a longtime Democratic consultant who is not working with any mayoral candidates this year.
Some New Yorkers were disappointed by the news that Weiner had apparently continued his online activities even after leaving office.
"I think he had a chance to redeem himself and if he did it twice, he really betrayed the public's trust again," said Jeremy Green, 22. "I think he's past the point of no return for New Yorkers."
The revelations come just two weeks after another scandal-scarred candidate, former New York Gov. Eliot Spitzer, announced his own attempt at political redemption. Spitzer, who resigned in 2008 after admitting to paying for sex with prostitutes, is running for city comptroller.
Weiner's problems began in May 2011, when a website run by conservative commentator Andrew Breitbart posted a photograph of a man's bulging underwear and said it had been sent from Weiner's Twitter account to a Seattle woman. Weiner denied he sent the photo, claiming his Twitter had been hacked.
But after more women came forward and more photographic evidence emerged, Weiner admitted he lied.
He then entered two years of self-imposed political exile, only to return this spring.
Under a huge media spotlight, he apologized repeatedly for his behavior in the initial days of his bid but then pivoted quickly into an issues-based campaign. He was largely well-received by voters and quickly established himself as a favorite in the race.
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