Incumbency and Partisanship in Preelection Poll Accuracy By Christopher P. Borick page4

The allocation of undecided voters plays a central role in analyses of preelection polls. Reporters regularly query pollsters on how undecided voters break during the final weeks and days of a campaign and look for direction on the impact of undecideds on election outcomes. To address these questions some common assumptions appear regularly in media accounts about polls. In particular, great weight is often placed on the percentage of the vote that an incumbent receives in the polls because it is assumed that undecided voters will break overwhelmingly to the challenger. Thus, no matter how large the spread between incumbent and challenger, incumbents polling below 50 percent can be considered in jeopardy of losing their seats. Studies of presidential elections support this contention, with incumbents’ poll numbers deviating by no more than two percentage points between 1980 and 2000.

However, in nonpresidential elections, the share of undecided voters that appears to go to incumbents is significantly greater than in presidential races, and, according to Bowers, this percentage appears to be rising over time. The evidence presented here supports the continuing decline of “the incumbent rule” regarding undecided voters. In the aggregate elections of 2004–6, challengers received slightly over half of unallocated voters in preelection polls.

Even more pronounced was the near even split of undecided voters between incumbents and challengers during the 2006 midterm elections. While incumbents themselves garnered about the same share of unallocated voters in 2006 as challengers, the interactive effect of partisanship and incumbency in this race appears to have been even more notable. More specifically, the aggregate incumbency-challenger split masked significant differences in how partisanship determined the break of undecided voters in 2006. This study finds that while Republican incumbents took a share (43 percent) of unallocated voters that was consistent with overall incumbent performances since 2000, Democratic incumbents actually took a majority (54 percent) of the undecided voters in 2006.

This study provides continuing evidence of the weakening of the “incumbent rule.” In nonpresidential elections, it is no longer viable to make claims that undecided voters will break solidly towards challengers, and that incumbents’ final results will match their preelection poll numbers. References made by both the media and pollsters to the incumbent rule are citing a phenomenon that no longer exists.

It may now be more valuable to complete a more comprehensive empirical analysis of the preelection polls to see if the break of unallocated voters may be tied to some “bandwagon” or “wave” effect that overrides any perceived challenger advantage. We might ask, for example, whether in years like 2006 and 1994 there was a tipping point where the overall strength of one party pushed undecided voters at a level equal to or greater than their knowledge and familiarity with incumbent candidates. To answer this question more empirical analysis will have to be undertaken. In the meantime, the “incumbent rule” deserves to go on hiatus.