In Obama Fundraising, Signs of a Shift From Online to In-Person

But several campaign finance consultants said in interviews this week that Obama is likely to use a wide range of tactics to manage expectations about how much he will bring in.

Anne Lewis, an Internet fundraising consultant who provided the strategy for Clinton's online fundraising effort, said campaigns estimate their budgets based on a wide array of clues from their online donors' past behavior.

The campaigns can measure how many people are opening their e-mail solicitations, and how many who open them ultimately contribute. That can help determine the most effective time of day to send solicitations, the most effective messages to use, even the best place to put the "contribute" button on the campaign's Web site.

"If they are committed to mining that data, they can learn a lot about how their audience responds," Lewis said.

But there is no way to control for every factor. Kristi Allison, 41, who owns a small business in Pinellas Park, Fla., went online 10 times over the course of a year to send Obama small amounts. Her last $25 contribution arrived in the Democratic presidential hopeful's campaign account on March 5. Then, her donations ceased.

Her decision had nothing to do with Obama.

"My feelings have not changed politically," she said. "I just have to worry about my business first. With gas over $4 a gallon, that's where my money is going."

Peter D. Greenberger, who oversees political advertising sales for Google, said he has seen the most online giving come when there are "surprising spikes in voter interest."

And it does not necessarily matter whether the interest is the result of good news or bad news for the candidate. Donations soared, for instance, when Clinton announced she would have to lend her campaign money, and when Obama lost the New Hampshire primary. The key, he said, is that people are compelled to donate because their candidate needs help.

"It's sort of like any news is good news," Greenberger said. "What we say to our clients is: If people are searching, you don't have the luxury of ignoring it."

One key difference between the primary season and the general-election contest is that those news-intensive periods are not likely to be as frequent, particularly during the summer.

For those drawing up general-election budgets, Lewis said, "the scary thing is, most of the money will arrive in September and October. You have to plan on ramping up to the bigger numbers in the fall, before you have concrete proof that the money will be there."

Database editor Sarah Cohen and research editor Lucy Shackelford contributed to this report.