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Polls Don't Reflect GOP's Real Chance of Taking Senate

Under-sampling.

8:25 AM, Oct 28, 2010 • By JEFFREY H. ANDERSON
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A close look at the Senate polls suggests that they are likely understating the probability of Republican victories. Most seem to be under-sampling either Republicans, independents, or both. As a result, in 10 of 12 key Senate races, the Republican candidate’s likelihood of winning appears to be greater than what the polls are registering.To be sure, such conjectures are as much an art as a science.  But this much we know: 

Polls Don't Reflect GOP's Real Chance of Taking Senate

Colorado Republican Senate candidate Ken Buck

First, both major parties’ ranks have thinned somewhat from 2004, while the pool of independent voters (Tea Party or otherwise) has expanded. Rasmussen shows 32 percent of voters now identifying themselves as not being a member of either major party, up from 24 percent in the fall of 2004.  That year, exit polling showed that 26 percent of voters were not affiliated with either party, and that number rose to 29 percent in 2008. It is likely to rise further. 

Second, in terms of party affiliation (aside from the growth in independents), we’re right back where we were in 2004. Both Gallup and Rasmussen now show Democrats enjoying an edge in party identification of just 1 to 2 percentage points — a spread that’s almost identical to what both polls registered in 2004. In that election, according to exit polls, turnout was split evenly between the two parties: 37 percent apiece.  

But by 2008, things had changed dramatically, as exit polling from 2008 showed a 7-point advantage in turnout for Democrats over Republicans. 

Since the Democratic advantage in turnout was 7 points higher in 2008 than in 2004, and since party identification is now essentially identical to 2004, a reasonable guess is that (all other things being equal), this year will be about 7 points worse for Democrats, in terms of turnout margin, than 2008 was. 

Thus, by adjusting the margin in each state by 7 points in the Republican direction, we can approximate the party split of 2004. And by increasing the percentage of independents by 10 percent, we can approximate their increased impact.

For example, in Wisconsin, 29 percent of voters in 2008 were independents. Increasing that number by 10 percent (29 times 1.10) gives us 32 percent (rounding to the nearest whole number).  And in 2008 the Democrats enjoyed a 6-point advantage over Republicans in the Badger State. Shifting that margin by 7 points leaves us with a Republican advantage of 1 point and gives us a projected turnout of 34 percent Republicans, 33 percent Democrats, and 32 percent independents, with 1 percent remaining (which can be added to the independent tally). Aside from the growth in independents, that projection is very close to 2004, when exit polls showed that 38 percent of Wisconsin voters were Republican, 35 percent were Democrats, and 27 percent were independents. 

For those who might wonder, the reason why this approach is more accurate than just using the 2004 split in party turnout for a given state and adjusting the percentage of independents upward, is that in some states there’s been a marked change in party affiliation across the past six years.  In West Virginia, for example, an 18-point gap between Democratic and Republican voters in 2004 actually shrank to 16 points in 2008, at the same time that the gap in party allegiance moved 7 points in the Democrats’ direction nationally.  In contrast, shifting the 2008 margin between Democrats and Republicans in each state by 7 points in the Republican direction allows the resulting estimate to reflect the 2004 turnout (nationally) while also taking account of unique developments in particular states.

In the states that are home to the most hotly contested Senate races, the projected turnout under this approach would favor the Democrats by an average of between 3 and 4 points — same as the Democrats’ average advantage in those states in 2004. 

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