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: