The centerpiece of it all is My.BarackObama.com, where supporters can join local groups, create events, sign up for updates and set up personal fund-raising pages. “If we did not have online organizing tools, it would be much harder to be where we are now,” Mr. Hughes said.

Mr. Obama, now the presumptive Democratic nominee, credits the Internet’s social networking tools with a “big part” of his primary season success.

The heart of the campaign’s online strategy is a teeming corner of Mr. Obama’s headquarters two blocks from the Chicago River, a crowded space that looks more like an Internet start-up company than a campaign war room. During a visit in late May, a bottle of whiskey sat, almost empty, atop a refrigerator (there had been plenty of victories to celebrate lately, a staff member explained).

Sitting amid a cluster of cubicles, Mr. Hughes, whose title is “online organizing guru,” handles the My.BarackObama.com site, which is known within the campaign as MyBo. Other staff members maintain Mr. Obama’s presence on Facebook (where he has one million supporters), purchase online advertising, respond to text messages from curious voters, produce videos and e-mail millions of supporters