Hal Malchow — who had previously approached the Democratic National Committee to propose using experimental controls to measure mail to voters but was repeatedly rebuffed — thought the Gerber-Green study was “the most important event in politics for a long time,” he says. “Eighty percent of what we’ve done in the past doesn’t work.” As the mail vendor for the A.F.L.-C.I.O., Malchow found a natural partner for his ideas in Mike Podhorzer, the organization’s deputy political director. Podhorzer saw Gerber and Green, who see themselves as researchers and not partisan advocates, as kindred spirits in a worldwide battle for knowledge between two camps he thought of as “gurus” versus “data.” As he says, “Until you get into a more rigorous approach, you are essentially left with what we had, which is that everything you did in a winning campaign was a good idea and everything that you did in a losing campaign was a bad idea.”

Podhorzer and Malchow began trying to adapt the Gerber-Green methods to the particular challenges faced by the A.F.L.-C.I.O., which regularly runs one of the largest independent campaign operations, almost always on behalf of Democrats. “Finding out the day after the election that Treatment A was the best is of limited value to an organization like ours,” Podhorzer says. “We’re actually trying to win the election.”

During the 2004 campaign, Podhorzer wanted to gauge voter reactions to his organization’s election messages in near-real time. A good poll shows how the electorate has moved over time, but it cannot isolate the effect of any individual appeal — and certainly not that of a single mailed leaflet, one of labor’s favorite tools for reaching member households. Focus groups offer a rich impression of how certain voters respond to that leaflet, but only the instant reaction of someone being paid $100 to have one. A focus group cannot say anything about whether a typical voter will even notice the brochure if it shows up in the mail wedged between a birthday card and a water bill.

Experiments provided a solution. The A.F.L.-C.I.O. planned to mail members monthly in 2004, and Podhorzer set out to design a “continuous feedback loop” testing different messages with small samples and then sending the most influential ones to a much larger target audience. As he examined the results, Podhorzer became even more frustrated with conventional polling. Asked if they would be more or less likely to vote for a candidate who favored shipping jobs overseas — a typical way of auditioning a promising line — voters across the board would tell pollsters that it made them “less likely.” But a draft leaflet about Bush’s policies had little impact on autoworkers who received it; they already knew what the union wanted them to think about the subject. Construction workers, however, didn’t know as much, and their minds changed. Experiments allowed Podhorzer to see which voters actually moved, not just count those who said they might.

Democrats have not been alone in experimenting with data-driven politics. As Dave Carney, once George H. W. Bush’s White House political director, prepared to guide the 2006 re-election campaign of Gov. Rick Perry of Texas, he invited Gerber and Green to conduct their experiments from within the campaign’s war room. Perry had spent more than $25 million to win a full term in 2002, much of it on broadcast advertising, and Carney thought a rigorous experimental regime could help “assure donors that we’re using their money as best as possible — spend it different, spend less of it.” Gerber and Green asked two political scientists who had informally advised George W. Bush’s 2004 re-election, James Gimpel and Daron Shaw, to collaborate on the project. Carney invited the quartet he called “our four eggheads” to impose experimental controls on nearly every aspect of campaign operations.

Perry won easy re-election in 2006, and their findings profoundly altered his 2010 tactics. Perry’s primary campaign this year sent out no direct voter-contact mail, made no paid phone calls, printed no lawn signs, visited no editorial boards and purchased no newspaper ads. His broadcast advertising strategy was informed by a 2006 experiment that isolated 18 TV media markets and 80 radio stations and randomly assigned each a different start date and volume for ad buys from a $2 million budget earmarked for the experiment. Public-opinion changes from the ads were then monitored with tracking polls. Carney estimates that the research saved Perry $3 million in this year’s primary campaign, and he still beat Kay Bailey Hutchison by 20 points. On Tuesday, the value of Perry’s unusually empirical approach to electioneering will be tested again, this time in a tough race against the Democratic nominee, Bill White, the former mayor of Houston.

After Bush’s 2004 re-election, Podhorzer invited other scientific-minded progressive operatives to A.F.L.-C.I.O. headquarters to share their research.Very few members would be recognizable to cable-news viewers; the group almost entirely bypassed the brand-name consultants whom campaigns like to unveil in press releases. “It’s not the big names on the door,” says Maren Hesla, who directed Emily’s List’s Women Vote! campaign. “It’s all the — God love them — geeky guys who don’t talk to clients but do the work and write the programs.”