Guest Pollster | August 17, 2010
Topics: Gallup , Generic House Vote , Interpreting polls , Likely Voters
If you heard a loud thump on Monday afternoon it just may have been the sound of worried Democrats hitting the panic button. That's when the latest Gallup weekly tracking poll was released and it showed Republicans with their largest lead yet on the generic ballot--7 points. It's the third consecutive week that Republicans have had a significant lead--following a 5 point lead two weeks ago and a 6 point lead last week. And that's among all registered voters, not just those likely to vote in November. Once Gallup begins screening for likely voters the GOP lead will almost certainly get larger since registered Republicans traditionally turn out at a higher rate than registered Democrats and this year Republicans are more enthusiastic about voting than Democrats.
But do Gallup's latest results actually mean that Republicans are likely to maintain a significant advantage on the generic ballot? Not necessarily. A closer examination of Gallup's weekly generic ballot data indicates that the current GOP advantage is likely to shrink over the next few weeks. In fact almost all of the week-to-week change in the standing of the parties appears to be due to random variation. There is little evidence of any real trend, at least so far.
Over the past 18 weeks, from April 12-18 through August 8-15, Republicans have received an average of 46% of the vote to 45% for Democrats on the generic ballot. There has been considerable week-to-week variation, from a 6 point Democratic lead only four weeks ago, to the current 7 point Republican lead, but no clear trend. Over this period, the correlation between the week of the survey and the size of the GOP lead is a very small and statistically insignificant .14.
Figure 1 displays both the week-to-week and the five week running averages for the Republican margin on the generic ballot between week 5 and week 14 of the Gallup weekly tracking poll. While the weekly average has shown considerable volatility, the five week running average has been fairly stable, fluctuating between a 2 point Democratic lead and a 2 point Republican lead with no clear trend.
The results in Figure 1 suggest that the weekly fluctuations in the generic ballot results are largely random. This conclusion is reinforced by the fact that there is a fairly large negative correlation of -.55 (p < .025) between the size of the GOP lead one week and the change in the size of that lead the next week. This means that the larger the GOP margin in a given week, the more that lead tends to shrink in the following week. These results again suggest that the week to week variation in the results is largely random.
Of course the fact that the current 7 point Republican lead on the generic ballot is likely to shrink doesn't alter the fact that Republicans are poised to make substantial gains in the midterm election. Even a tie on the generic ballot, given normal turnout patterns, is good news for the GOP. So while it may not be time yet for Democrats to hit the panic button, there is plenty of reason for them to be worried.