A few hours after lunch last week with his old buddy President Bush, the Irish rock star Bono, a guitar slung low over his hips, paused near the climax of a raucous U2 concert to ask 20,000 fans to sign up to save the world.

From a mammoth stage in a downtown Washington arena, Bono asked his audience to lift their cell phones and send a text message that would connect them to an organization he helped launch last year to prod the United States toward greater efforts against poverty and disease in the developing world. Instantly, the hall filled with the spectral glow of upraised phones, the way it once might have filled with lighters held by fans cheering the band.

By the time U2 was on its second encore, the names of those who had enlisted in the cause were streaming across a giant video screen above the stage. And before the fans filed out, those who signed up already had received a text-messaged "thank you" from Bono on their phone.

So ended another night's work in a campaign to create something new in American politics: a nationwide, grass roots constituency to aid the world's neediest people.

The organization that Bono asked his fans to join is called "ONE: The Campaign to Make Poverty History." It was formed last year by 11 leading anti-poverty and charitable organizations, from CARE and World Vision to Bono's own group, DATA.

Moral authority has never been a problem for these groups on Capitol Hill. What they have lacked is political clout. Other than venerable but limited church networks, development advocates have never built a mass community of voters for their cause. ONE wants to change that.

"Any time we would go to legislators, we'd always hear back: 'This is great, but I have no voters in my district who care about this,' " said Raymond Offenheiser, the president of Oxfam America. "After hearing that for 20 years, we finally woke up."

In less than two years, ONE has built an e-mail list of about 2 million names, an incredibly rapid expansion. At last week's concert, Bono told the audience the group expected to reach 5 million by 2008; at that level, he noted pointedly, it would be larger than the National Rifle Association.

ONE's explosive growth testifies to the power of celebrity, the power of the Internet, and perhaps to the power of an idea — a global offensive against extreme poverty and debilitating disease — whose time has come. But the organization also is an unproven experiment whose fate may offer important clues about what it takes, in the era of Internet politics, to transmute fleeting empathy into enduring commitment, and online fervor into on-the-ground leverage.

The group has been creative and relentless in promoting its mission. Millions have learned about it through stylish television, magazine and online public service announcements featuring not only celebrities, such as Brad Pitt and George Clooney, but unexpected allies, like conservative televangelist Pat Robertson.

On U2's fall tour across America, Bono has delivered his cell phone appeal at every stop, yielding as many as 10,000 names a night.

From all these sources, ONE directs its prospective recruits to its Web site (www.one.org). Once there, they are asked not to contribute money, but to sign the group's petition urging the United States over time to slightly more than double its spending on foreign aid.

Each time someone signs, the group identifies another name that can be recruited for future lobbying campaigns. That process has allowed ONE, like other Internet-based organizations, to amass a network far faster than earlier generations of advocacy groups that relied on direct mail and phones.

But the challenge for advocacy groups now is to transform a large e-mail list into a functioning membership; as Bono probably understands, the NRA is effective less because of the number than the commitment of its members. ONE has tested its network only a few times. Last summer, half a million supporters signed letters urging Bush to seek a generous package of African aid at the G-8 meeting. In July, 25,000 people from the network called the Senate to support an amendment increasing by $100 million U.S. support for international efforts against AIDS and malaria.

That response was slight compared to the letter-writing campaign, but vigorous next to the grass roots support usually generated for international aid.

Shrewdly, ONE is also hiring organizers to construct networks of local volunteers; last summer it took 100 of the volunteers to the G-8 meeting and then arranged for them to speak at home about their experiences.

That may be the group's most promising idea yet. Even with the Internet's remarkable capacity to gather like-minded people, and pitchmen as appealing as Bono and Pitt, the most durable way to change public opinion in America is still neighbors talking to neighbors. Building a constituency for foreign aid won't be easy. But it is more likely to come together across a back fence than in a concert hall.

Ronald Brownstein is a national political correspondent for the Los Angeles Times.