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 Published in Science, the study was retracted when the data forgery came to light.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.

To be sure, there are some unique features of the gay-rights fight that other causes don’t share. “An advantage we have is that we are in every family,” Solomon said. “Dick Cheney never has to deal with a poor person, but he has a lesbian daughter. That makes a big difference.” Gay rights also didn’t pose a threat to anyone’s economic interests, unlike many lefty priorities—but, Solomon points out, the marriage campaigners were up against well-financed opponents on the religious right, including evangelicals and the leadership of the Catholic and Mormon churches.

Solomon, a former Republican congressional staffer from the Midwest, got his start fighting for gay rights in Massachusetts. In 2003, the state supreme court ruled that gays should be allowed to marry, and a bipartisan group of state lawmakers—from Republican Governor Mitt Romney to many Italian-American Catholic Democrats—were determined to stop it from happening by amending the state constitution. That was what happened in Hawaii in the 1990s, after a court similarly ruled in favor of gay marriage: Legislators put a constitutional amendment on the ballot to ban gay marriage, and voters overwhelmingly approved it.

Solomon’s employer at the time, MassEquality, faced a seemingly impossible task: To prevent the issue from going to the ballot, they had to get 75 percent of the state legislature on their side. In order to put grassroots pressure on lawmakers, the gay-rights campaigners got creative: They combed through state records to find the hundreds of gay couples who had already gotten married, then sent each one a postcard asking if they’d be willing to talk to their legislator. This tactic paid off in a powerful way when a pair of lesbians in rural Massachusetts befriended their state representative and helped change his mind about the issue. In another case, when their research found that a certain lawmaker loved musicals, the campaigners got the author of Wicked to write him a personal letter comparing the main character’s underdog struggle for acceptance with the situation gay people faced.