Then, as Obama marched toward the presidency, a new suggestion emerged: That he was not eligible to serve.

That theory first emerged in the spring of 2008, as Clinton supporters circulated an anonymous email questioning Obama’s citizenship.

“Barack Obama’s mother was living in Kenya with his Arab-African father late in her pregnancy. She was not allowed to travel by plane then, so Barack Obama was born there and his mother then took him to Hawaii to register his birth,” asserted one chain email that surfaced on the urban legend site Snopes.com in April 2008.

Another early version of the theory, reported by the Chicago Tribune in June 2008, depended on a specious legal theory that was, for a time, the heart of the argument: that Obama was born in Hawaii but had a Kenyan father, and his mother was only 18 years old. Therefore, under existing immigration law, he was not eligible for automatic citizenship upon birth — a claim that depended on an understandable, but incorrect, reading of immigration law. Other theories suggested that Obama lost his U.S. citizenship when he moved to Indonesia or visited Pakistan in violation of a supposed State Department ban as a young man. (There was no such ban.)