Answer: a very small number of American citizens, most of whom are uninterested in politics. Startling New Yorker piece by Elizabeth Kolbert.
According to a recent Washington Post/ABC News poll, just six per cent of Americans—or less than one-sixteenth of the electorate—think there’s a good chance that they will change their minds about the Presidential race before November. Only nineteen per cent of those polled said there was any chance they’d change their minds. For comparison’s sake, at a similar point in the 2008 election cycle, ten per cent of Americans said they were undecided, and twenty-five per cent said there was a chance they’d switch their choice. Former Clinton adviser Paul Begala recently noted in Newsweek that when you factor out the undecideds in securely red or blue states (since their votes won’t change the Electoral College results), the election comes down to “around 4 percent of the voters in six states.”
“I did the math so you won’t have to,” Begala continued. “Four percent of the presidential vote in Virginia, Florida, Ohio, Iowa, New Mexico, and Colorado is 916,643 people. That’s it. The American president will be selected by fewer than half the number of people who paid to get into a Houston Astros home game last year.”