The marriage campaigners spread their message using a sophisticated persuasion campaign—a tactical innovation that many others are now trying to emulate. Armies of canvassers—both paid workers and volunteers—set out to have in-depth conversations with thousands of voters using ideas developed with help from the liberal Analyst Institute, a quasi-academic campaign-tactic lab. Rather than parroting a script, the canvassers used a few open-ended prompts (“What does marriage mean to you?”) and drew on their own experiences to have long conversations about family and faith that often turned personal—and changed people’s minds.

These techniques came in for some scrutiny recently with the controversy over a study by the political scientists Michael LaCour and Donald Green that turned out to be based on fake data. The study purported to show a huge, lasting effect on people’s opinions about gay marriage when they had a personal conversation with a gay canvasser—but not a straight one. Published in Science, the study was retracted when the data forgery came to light. Had it been real, the study would have provided the first academic proof of the kind of techniques the gay-marriage campaigners pioneered. But Solomon and other advocates say they have plenty of rigorously field-tested evidence from their work that these techniques do work. (There’s no evidence, however, that a canvasser has to be gay to have an effect.) And other campaigners, notably abortion-rights advocates, are already putting similar tactics to work.