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.