But this story is even better than just her getting booed. Once they boo her, she tells them she doesn't care who they vote for, as long as they get out and vote. Do you think she would have said that had they cheered the Obama comment? This is another classic example of the liberal "bubble" mentality.
Madonna lives in the liberal bubble where everybody thinks like her. It was beyond her comprehension that many in the audience were supporting Romney. And when she realized it, she immediately tries to reverse course.