Campaigns are not required to disclose detailed information on donors who contribute less than $200, so little is known about the smallest givers. But campaigns do report information on small donors once their combined contributions top the $200 mark.

One such donor is Timothy Sweeney. The 24-year-old medical student at Duke University first noticed Obama when Sweeney was an undergraduate in Chicago, and liked his "high-minded approach to things." Sweeney has donated online in small increments adding up to about $300 so far, and says he may give $100 to $150 more if Obama makes it to the general election.

Obama, says Sweeney, strikes him as "just an honest, decent man, and I felt like somebody like that should be in the race."

Obama also appears to draw a disproportionate amount of support from black donors. In ZIP codes where 90 percent or more of residents are black, the AP analysis found, Obama attracted nearly $150,000 from individuals who gave small donations totaling at least $200, compared with less than $20,000 for Clinton and just $2,140 for McCain.

Obama gets 20 percent of his campaign dollars from the biggest donors, those contributing the maximum $2,300 for the primary campaign, compared to 34 percent for Clinton and 39 percent for McCain, according to the private Campaign Finance Institute.

While little is known about the characteristics of Obama's smallest donors, the impact of their giving is unquestioned.

Their combined purchasing power has turbocharged Obama's campaign, allowing him to do virtually everything he wanted in state after state in the prolonged Democratic duel with Clinton. They also have given Obama the luxury of spending more time talking to the public and less attending fundraisers, and have created a host of supporters working to elect him.

"Anybody that contributes, we immediately call them and ask them if they would like to be part of our organization," says Obama campaign manager David Plouffe. "Every state we go into, we have a foundation of support."

Not only can Obama keep returning to his donors for repeat contributions — only 2 percent have given the maximum $2,300 — he still has the potential to increase his pool of contributors from the names on his 3-million-plus e-mail list of contacts. Plouffe stresses that "we don't view our online community as an ATM machine," rather as a network of supporters ready to help in all sorts of ways.

Michael Malbin, executive director of the Campaign Finance Institute, said even the smallest contribution helps voters feel they have a stake in the campaign. Obama, he said, has taken to heart a lesson taught by Saul Alinsky, the father of community organizing, who often spoke about the importance of getting people to contribute even as little as 50 cents to get them invested in a cause. (Obama began his work as a community organizer in Chicago in 1985, more than a decade after Alinsky died, but studied Alinsky's methods.)

"Once a person does anything, that person is likely to do some other thing," Malbin said. In that respect, Malbin said, Obama's small donors are dramatically different from those of Howard Dean, the 2004 Democratic candidate who first tapped into small giving over the Internet but was unable to translate that support into votes.

"It's not just about getting the small gift," said Malbin. "It's about bringing a new person into the campaign, both financially and in terms of the volunteer program, and turning out the vote."

At least 20 percent of Obama's donors never have given to any candidate before, according to Plouffe.

Bonnie Reagan, a 56-year-old consultant from Nashville, is an example. Obama is the first candidate she's ever given to — more than a dozen contributions so far totaling somewhere under $1,000. And after she gave, she took the campaign up on its invitation to help, and ended up working a phone bank during the early primaries.

Gerald Cook, a 67-year-old retired aerospace engineer in Denver, has $25 for Obama automatically deducted from his checking account each month and then tosses in "a little on top of that." He helped out on the Obama campaign in the lead-up to the Colorado caucuses.