The New York Times explains it this way: “I’m going to need you just as fired up as you were in 2008,” Mr. Obama told them. When a heckler yelled “You’re a liar!” in response, people nearby spent the next few minutes shouting the heckler down, making it difficult for many in the audience to hear what the president, who was flanked on the dais by Maryland’s top elected Democrats, was saying. Now see it happen: