One of the theories raised to explain the problematic New Hampshire primary polls is a late shift, perhaps on the heels of the Hillary Clinton "tears" story, that polls missed either because they stopped interviewing or completed the bulk of their calls on Sunday or earlier. Two prominent pollsters say such as shift is unlikely given one result in the exit polls.
For example, Andrew Kohut, writing in his must-read op-ed in yesterday's New York Times, concludes:
Yes, according to exit polls the 17 percent of voters who said they made their decision on Election Day chose Mrs. Clinton a little more than those who decided in the past two or three weeks. But the margin was very small — 39 percent of the late deciders went for Mrs. Clinton and 36 percent went for Mr. Obama. This gap is obviously too narrow to explain the wide lead for Mr. Obama that kept showing up in pre-election polls.
Gary Langer, director of Polling for ABC News, agrees in a lengthy review posted this morning:
[T]he exit poll asked voters the time of their decision. Seventeen percent said they decided on Election Day; they voted for Clinton over Obama by a 3-point margin, 39 to 36 percent – hardly a significant swing from the overall result (Clinton +2). Those who said they decided in the previous three days, 21 percent, favored Obama over Clinton by 3 points, 37-34 percent – further deflating the late-decider argument. Those who decided previously, 61 percent of voters, favored Clinton over Obama by 41-37 percent.
For reference, here is the exit poll data that Kohut and Langer cite:
But wait. Putting aside the issue of whether respondents can accurately recall when they reached a decision, why does the absence of a big Hillary bump among the late deciders rule out the possibility of some late shift away from Obama? Keeping in mind where Clinton stood in polls before Iowa, we would not be looking for a Clinton bump as much as an Obama "unbump" (to paraphrase the comment left yesterday by my friend reader Mark Lindeman).
Let me explain. Start by looking at our chart of the polls conducted in New Hampshire during 2007. Our trend estimate shows Clinton winning between 35% and 40% of voters between June and early November. As always some individual polls were a little higher, some lower. Her support declined slightly in December, to an average of about 32%, with the usual variation slightly higher and lower. In December, the undecided category averaged 11% and the support for Richardson, , Biden, Dodd and Kucinich was roughly twice what they received on Election Day. Give Clinton a proportional share of the undecideds and those that moved away from the single digit candidates, and she was headed to roughly 37% of the vote. Thus, putting aside whatever happened in the weekend after Iowa, Clinton would not have needed any massive last minute gains to get to the 39% she received in the final count.
Now on the other hand, if Obama had surged after Iowa to a double digit win as the polls seemed to predict, we certainly would have expected to see late deciders favoring him heavily. But if that weekend bump collapsed (or if it never existed in the first place)? In that case, we would not expect to see much of a difference between early and late deciders. Clinton needed only a modest increase in support to reach 39% of the vote.
Consider a historical example. In 1984, Gary Hart surged to victory along a similar trajectory as what polls seemed to forecast for Obama last week. According to the ABC News/Washington Post polls compiled by Samuel Popkin in his book, The Reasoning Voter, Hart trailed Mondale 13% to 37% just before the Iowa Caucuses (with John Glenn running second (at 20%). The Iowa Caucuses were held on a Monday in 1984, a full week before the New Hampshire primary. On interviews conducted Wednesday through Friday after Iowa, Mondale's numbers held constant (38%) while Hart moved up (to 24%). Over the course of the week, Mondale steadly lost support while Hart continued to rise until moving ahead on the Monday before the primary by an eight point margin (35% to 27% -- although ABC reported a three-day rolling average at the time showing the candidates tied). The next day, Hart defeated Mondale, 37% to 28%.
The ABC exit poll also asked voters when they made up their mind, and the pattern is what we would expect. Among those who made up their minds over the final weekend or the two days just before the primary, Hart led Mondale by a four-to-one margin (56% to 14%).
So let's consider a hypothetical question: What would have happened if late deciders had broken for Barack Obama by the same margins as they preferred Hart to Mondale in 1984? If I go to my spreadsheet, play "what if" and imagine that late deciders -- those who made up their minds over the last three days -- had preferred Obama 53% to 20% (while all other preferences held constant), Obama would have defeated Hillary Clinton by 10 points (43% to 33%).
Of course, none of this explains exactly what happened last week, and the time-of-decision exit poll data is just one small piece of the puzzle. Opinion polls may have accurately measured a post-Iowa bounce for Barack Obama that "unbounced" over the last 24 hours, or the apparent surge may have been an artifact of some survey error (or perhaps some combination of both). But either way, the lack of a difference between late and early deciders does not tell us much. It certainly does not preclude -- by itself -- the possibility of shift of voters to Obama on Saturday and Sunday that shifted back to Clinton on Monday.
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