Pollsters, in turn, treat the meaningless answer as though it represents what voters at large are thinking. But the undecided voters at large have not made a meaningless choice, because no one has forced them to do so. The net result is that in the months leading up to the election, polls misrepresent what the voters at large are really thinking.
Early on in the campaign, how do the undecided voters in the poll samples come up with a choice? Typically, the name they choose is the one they have heard of most frequently, even though they are not necessarily paying close attention to the candidates and are not necessarily committed to the choice they give the pollsters. Indeed, pollsters will readily admit that early polls do not reflect voters' real choices as much as these polls reflect the higher name recognition or greater media coverage of one of the candidates.
And that gets back to the convention bounce. During media coverage of the Democratic convention, it may well be that undecided voters in poll samples, when pressed for a candidate choice, will choose Obama, because of the positive news about him during that time (and probably negative news about McCain). Similarly, during media coverage of the Republican convention a few days later, the undecided voters in brand-new poll samples are likely to mention McCain when pressed for a choice. In neither case do the polls tell us the truth about the undecided voters, perhaps most of whom may not make up their minds on the basis of the conventions at all.
If pollsters want to accurately measure the effect of the conventions, they should allow undecided voters to admit their indecision, thus allowing us to see if people have—of their own free will—moved from one candidate to another. But pollsters are so locked in to the standard, forced-choice vote question, they will never do that. Instead, they will "discover" an apparent fickle electorate, flitting from one candidate to the other.
Indeed, Gallup's Jeff Jones has documented the history of past bounces, ranging from -1 percentage point (for John Kerry in 2004) to +16 (for Bill Clinton in 1992), with an average of 5 points for Republicans and 6 points for Democrats. This suggests an average swing in the difference between the two candidates' support of 10 to 12 percentage points. But don't believe it. The bounce is more likely a reflection of the polls' samples than the electorate at large.
Ultimately, as Election Day nears, the question about which candidates voters would support if the election were held "today" becomes more and more appropriate, because the election is almost "today." The poll samples will slowly converge with what voters at large are really thinking, the size of the real undecided vote at large will decline to resemble what the polls say, and in the end, polls will usually end up with final pre-election polls fairly close to the election results. Pollsters will then trumpet their "success" in measuring the will of the voters, ignoring how wrong and contradictory their polls have been throughout the campaign season.
David W. Moore is a former vice president of the Gallup Organization and managing editor of the Gallup Poll. He is the author of The Opinion Makers: An Insider Exposes the Truth Behind the Polls (Beacon Press).