Three months before Representative Anthony D. Weiner sent a photo from his Twitter account to a 21-year-old Washington State college student named Gennette Cordova, a small group of determined, self-described conservatives was warning young women on Twitter, including Ms. Cordova, to be wary of him.

Calling themselves the #bornfreecrew on Twitter, members of the group closely monitored those whom Mr. Weiner was following, taking it upon themselves to contact young women they believed to be “schoolgirls,” and urging them publicly to stay away from him, according to an analysis of posts on Twitter’s public stream.

By early May, members of the group were also speculating that Mr. Weiner would be caught in a sex scandal. The leader, a man who identified himself on Twitter as Dan Wolfe and used the handle @PatriotUSA76, is the same Twitter user who discovered the photograph that Mr. Weiner took of himself and sent to Ms. Cordova. He shared it with his followers and the conservative blogger Andrew Breitbart, who made it public the next day.

As Democrats and Republicans embrace Twitter and other social media tools as a way to interact with their constituents and woo voters, many have discovered a downside to online communication: cyberstalkers, who track and criticize their every move.