Simply identifying a person as a voter, as many volunteers did — “Mr. Jones, we know you have voted in the past” — acts as a subtle prompt to future voting, said Dr. Cialdini, a foundational figure in the science of persuasion. “People want to be congruent with what they have committed to in the past, especially if that commitment is public,” he said.

Many volunteers also asked would-be voters if they would sign an informal commitment to vote, a card with the president’s picture on it. This small, voluntary agreement amplifies the likelihood that the person will follow through, research has found.

In a now classic experiment, a pair of Stanford psychologists asked people if they would display in a home window a small card proclaiming the importance of safe driving. Those who agreed to this small favor were later much more likely to agree to a much larger favor, to post a large “Drive Carefully” sign on their lawn — “something no one would agree to do otherwise,” Dr. Cialdini said.

Obama volunteers also asked people if they had a plan to vote and if not, to make one, specifying a time, according to Stephen Shaw, a retired cancer researcher who knocked on doors in Nevada and Virginia in the days before the election. “One thing we’d say is that we know that when people have a plan, voting goes more smoothly,” he said.

Recent research has shown that making even a simple plan increases the likelihood that a person will follow through, Dr. Rogers, of Harvard, said.

Another technique some volunteers said they used was to inform supporters that others in their neighborhood were planning to vote. Again, recent research shows that this kind of message is much more likely to prompt people to vote than traditional campaign literature that emphasizes the negative — that many neighbors did not vote and thus lost an opportunity to make a difference.

This kind of approach trades on a human instinct to conform to social norms, psychologists say. In another well-known experiment, Dr. Cialdini and two colleagues tested how effective different messages were in getting hotel guests to reuse towels. The message “the majority of guests reuse their towels” prompted a 29 percent increase in reuse, compared with the usual message about helping the environment. The message “the majority of guests in this room reuse their towels” resulted in a 41 percent increase, he said.