It would be a political miracle if the Democrats did not lose seats in the 2010 Congressional elections, yet the polls so far suggest that scenario is doubtful at best. I think it's because most polls are providing a rosier picture for the Democrats by reporting voting intentions of the general public, or registered voters, rather than the much smaller segment of "likely voters" that will ultimately turn out to cast a ballot.
That the Democrats will almost certainly lose House seats in 2010 is attested to by several factors. The most important, of course, is that since the advent of the current two party system (Republicans and Democrats), the party of the president almost always loses seats in a mid-term election. The best theory for this phenomenon is that disgruntled people (i.e., those who identify with the "out" party) are more motivated to cast a protest vote than the relatively satisfied people (i.e., those who identify with the party of the president) are to cast a vote of support.
The second factor is that, in the wake of the protracted war in Iraq and the sagging economy, Democrats won many seats in 2006 and 2008 that would "normally" go to Republicans. In 2010, with Bush gone and a Democratic administration in charge, Democratic House members in those "normally" Republican seats are going to be quite vulnerable.
The final factor is that as a general rule, Republicans are more likely to turn out than Democrats, because Republicans tend to be higher on the socio-economic scale – generally more educated, with higher incomes, and more actively involved in politics than Democrats.
So, if all of these reinforcing factors suggest the Republicans are likely to gain seats, why aren't the polls showing that? After the jump, some interesting recent poll results (see pollingreport.com).
July-August Polls 2009 Measuring Support for Congressional Candidates, 2010
|July 31-Aug 1
|CNN/Opinion Research Corp||44||34||22||+10|
|NBC/Wall Street Journal||46||39||15||+7|
|NPR/POS and GQRR||42||43||15||-1|
|GWU – Tarrance/Lake||43||40||17||+3|
Note that there is little difference in the lead that polls show for Democrats when the sample is either the general public or registered voters – from six to ten percentage points. However, the two polls that reported results based on "likely voters" show essentially a dead heat (a 3-point Democratic lead or a one-point Republican lead).
Nate Silver (at fivethirtyeight.com) suggests caution in relying on likely voter models this early in the 2010 campaign. Generally, I agree that early polls-- especially in specific races (as opposed to the more general generic ballots reported above)-- need to be viewed with caution. Many people are undecided 10 to 12 months ahead of the election, though some pollsters obscure that fact by using a forced choice format. See, for example, the contrast between Diageo/Hotline and Gallup above, the former showing 30 percent of registered voters undecided, Gallup showing just 7 percent.
Furthermore, different polling organizations use different screeners to arrive at their presumed "likely voters," with some more "aggressive" than others. So, it's difficult to make direct comparisons with polls showing different leads, even if they base their results on likely voters, rather than registered voters or the general public.
That said, I would argue that in general we get a more realistic view of the general sentiment of voters, if the sample has been screened fairly tightly to produce a relatively small segment of likely voters rather than a much larger group of people-- the general public or even "registered voters." In mid-term elections, turnout is only about half or so of turnout in presidential elections. Thus, screening out the non-voters is much more sensitive for understanding mid-term elections than presidential elections.
So, contrary to Nate Silver's advice, I would suggest that when polls diverge, one based on likely voters is probably a better reflection of the actual electorate than a poll based on the general population or even registered voters.
(In Part II I will discuss the exceptions to the general rule that the president's party loses House seats in mid-term elections, and whether those exceptions are relevant to 2010.)