Michael Erard recounts the big role that small donors have played in political campaigns:
For Jimmy Carter, 38 percent of his money came in small donations; it was 40 percent of Gerald Ford’s. Amazingly, it was 60 percent for Ronald Reagan in his 1984 re-election. Yet in the informational antediluvian era of 1976 and 1984, obtaining that money was time-intensive and expensive. Hitting up a donor the second time cost just as much as the first. The Internet changed all that, providing tools that Howard Dean pioneered in 2004, but which Barack Obama rolled out far more extensively in 2008. Obama hits you up more than once because it costs him next to nothing to do so.
And repeat calling works:
In 2008 slightly more than half of Obama’s donors who gave an aggregate of $200 or more started out giving amounts less than $200. In other words, the Obama fundraisers were able to up-sell a substantial percentage of givers.
A joint paper (pdf) from CFI, Brookings, and AEI found that "giving small amounts seems to heighten nonfinancial forms of participation by people who feel more invested in the process." But Erard fears that campaigns may be alienating exactly those donors by repeatedly asking them for more money:
The old small donor is someone who gives money because he or she’s politically active and ideologically determined. The new small donor gives in order to have a stake in the political system; they become ideological because they’ve given money. Here’s the problem: bombarding previous donors until they give again is a strategy designed with the old small donor in mind. Nobody knows what it will do to the new small donor, whose participation in the system is fragile.
Over at the Billfold, a fundraiser answered questions anonymously. On why Obama might email you even if you gave yesterday:
Each email solicitation uses a new set of data. Sometimes the criteria is as simple as “past donors who have not given this fiscal year.” Sometimes, it’s more specific, like, “donors who are women with a birth year between 1976 and 1991 with a preferred mailing address in California who have given between $1 and $249 in the past six months, but have not made a donation in the past 30 days.”
Segmenting donors like this is far more effective. It raises more money, and reduces the number of people who click the “unsubscribe” link. It’s also far more work. Besides pulling new sets of data, smart fundraisers also proof data to make sure it worked correctly. Depending on the size of the organization and the workload, data is pulled and proofed hours, days, or even a week in advance. If you donate after that, you’ll still receive the solicitation.