When you look at how full your spam folder is and how many canned emails you likely delete without reading it may surprise you that, according to Businessweek (hat tip: Larry), "[m]ost of the $690 million Obama raised online came from fundraising e-mails." What makes people open and respond to mass emails? The Obama campaign apparently studied the issue in great detail. They sent out emails to test groups changing subject lines and formatting and carefully studying how which emails generated the most cash.
The subject lines "I will be outspent" and "some scary numbers" were the big winners, generating five times as much cash as "the one thing the polls got right..." The above table seems to lead to the conclusion that Democrats respond much more strongly to appeals to anxiety ("scary numbers") than to appeals to reason.
It is important to know your audience.