The appeals were the product of rigorous experimentation by a large team of analysts. “We did extensive A-B testing not just on the subject lines and the amount of money we would ask people for,” says Amelia Showalter, director of digital analytics, “but on the messages themselves and even the formatting.” 

The campaign would test multiple drafts and subject lines—often as many as 18 variations—before picking a winner to blast out to tens of millions of subscribers.  

It quickly became clear that a casual tone was usually most effective. “The subject lines that worked best were things you might see in your in-box from other people,” Fallsgraff says. “ ‘Hey’ was probably the best one we had over the duration.” Another blockbuster in June simply read, “I will be outspent.” According to testing data shared with Bloomberg Businessweek, that outperformed 17 other variants and raised more than $2.6 million. 

Writers, analysts, and managers routinely bet on which lines would perform best and worst. “We were so bad at predicting what would win that it only reinforced the need to constantly keep testing,” says Showalter. “Every time something really ugly won, it would shock me: giant-size fonts for links, plain-text links vs. pretty ‘Donate’ buttons. Eventually we got to thinking, ‘How could we make things even less attractive?’ That’s how we arrived at the ugly yellow highlighting on the sections we wanted to draw people’s eye to.” 

Another unexpected hit: profanity. Dropping in mild curse words such as “Hell yeah, I like Obamacare” got big clicks. But these triumphs were fleeting. There was no such thing as the perfect e-mail; every breakthrough had a shelf life. “Eventually the novelty wore off, and we had to go back and retest,” says Showalter. 

Fortunately for Obama and all political campaigns that will follow, the tests did yield one major counterintuitive insight: Most people have a nearly limitless capacity for e-mail and won’t unsubscribe no matter how many they’re sent. “At the end, we had 18 or 20 writers going at this stuff for as many hours a day as they could stay awake,” says Fallsgraff. “The data didn’t show any negative consequences to sending more.”