Jonathan Zimmerman teaches history and education at New York University. He is the author, most recently, of Small Wonder: The Little Red Schoolhouse in History and Memory.

The Internet can be a scary place. Troll some of its darker corners, and you’ll find plenty of people who think that George W. Bush knew about the Sept. 11, 2001 attacks before they happened. The Israelis were supposedly tipped off, too: That’s why Jews went to work late that day. And so on.

Now imagine that one of these conspiracy theorists announced that he was running for president. And suppose further that many other candidates and leading party figures said they didn’t believe the 9/11 plot, but that others were free to make up their own minds.

We would be outraged, of course. In the face of a fantastic lie, simply stating a personal preference – and leaving it at that – gives indirect confirmation to the lie itself. It becomes a matter of taste, like one’s favorite color or flavor of ice cream.

That’s the dirty little game that many Republicans have been playing with the so-called “birther” screed, which holds that President Obama was born in a foreign country or otherwise is not a “natural born citizen,” making him ineligible to be president. And they could get away with it, because no GOP presidential hopeful had personally endorsed the birther idea....