In words that he has come to regret, Barack Obama opined as to why he was having a hard time winning over many blue-collar voters: "They get bitter, they cling to guns or religion or antipathy to people who aren't like them or anti-immigrant sentiment or antitrade sentiment as a way to explain their frustrations."

It was a throwaway line to a private audience at a San Francisco fund-raiser. And it was made public on a liberal Internet blog, not by right-wing commentators. But Mr. Obama's opponents seized on the quote. It was...