It was nearly 9 p.m. on a Friday when Ms. Cordova, who was preparing to head out for the night with a friend, logged onto Twitter and discovered that Representative Anthony D. Weiner had sent her a suggestive photo of himself in gray boxer briefs.

“It didn’t make any sense,” Ms. Cordova, a 21-year-old college student in northwestern Washington State, said in her first extensive interview since Mr. Weiner confessed in a news conference Monday to sending her the photo. “I figured it must have been a fake.”

Ms. Cordova’s experience with Mr. Weiner appears to fit a pattern: in rapid and reckless fashion, he sought to transform informal online conversations about politics and partisanship into sexually charged exchanges, at times laced with racy language and explicit images.

Ms. Cordova, who had traded messages with Mr. Weiner, a New York Democrat, about their shared concern over his conservative critics, said she had never sent him anything provocative. Asked if she was taken aback by his decision to send the photo, she responded, “Oh gosh, yes.”

Ms. Cordova spoke to The New York Times as Mr. Weiner faced intensifying calls for his resignation because of his acknowledged online sexual communications with at least six women over the last three years. On Wednesday, House leaders began a concerted effort to persuade Mr. Weiner to step down, worried that the sensational coverage of his online sexual liaisons had created political chaos and was subjecting the Democratic Party to ridicule.

The women who have acknowledged encountering Mr. Weiner on social media and then having personal communications with him varied in age, race and location, and even in their willingness to engage the congressman in sexual discourse. But in each case Mr. Weiner’s online conduct in many ways mirrored that of his offline life — he was aggressive, blunt, feisty and willing to push boundaries with an apparent disregard for the possible consequences.

The women came to his attention after he had come to theirs. Usually they were admirers of his scrappy progressive politics and youthful energy, and either posted an enthusiastic comment on his Facebook page or sent him an admiring Twitter message.

Ms. Cordova was first impressed with Mr. Weiner after she saw him take on Representative Michele Bachmann, a Tea Party-backed Republican from Minnesota, on Fox News’s “Hannity.” Ms. Cordova and her boyfriend thought the congressman was smart and funny, and they both started following him on Twitter.

“I tweeted words of support for him as a politician, and I retweeted his tweets often beginning around early to mid-April,” Ms. Cordova said, in a series of conversations by phone and e-mail over the past two days.

She added that in mid-April, “he thanked me for the support” using a direct message — a private note sent via Twitter — and he then signed up as her follower on Twitter, meaning that he could easily read all of her posts.

Ms. Cordova said that after Mr. Weiner began following her, critics of the congressman started sending her harassing messages. She said she then began communicating, always electronically, with the congressman about their shared annoyance with those critics.

Ms. Cordova provided a portion of her communications with Mr. Weiner to The Times, in which they messaged back and forth about the online detractors and their tactics. But Ms. Cordova would not make all of her interaction with him available for review.

“I have not sent him any suggestive messages,” Ms. Cordova said.

She said she was, however, surprised by his informal tone. “He was just very casual,” she said. “It wasn’t like talking to a U.S. congressman.”

A spokeswoman for Mr. Weiner did not dispute Ms. Cordova’s account.

Mr. Weiner, at his news conference on Monday, said he had sent Ms. Cordova the underwear photo “as part of a joke.” But Ms. Cordova said the image was not in keeping with the tenor of their previous interactions.

“I still didn’t get the joke part of it,” she said.