Rating How Competitive New Jersey Congressional Districts Are: Nate Silver’s PPI Index

Nate Silver of FiveThirtyEight.com has just introduced his new “Partisan Propensity Index” (PPI). If you’ve been following elections closely, you’re probably already familiar with the Partisan Voting Index (PVI) from Cook, and similar statistics from Swing State Project. Cook’s idea is to look at how each Congressional District voted for President compared to the nationwide average. So, for example, the NJ5 district (Garrett’s) is rated R+7, meaning it voted 7 points more Republican than nationwide, while NJ13 (Sire’s) is rated D+21. You can see why Democrats had such a hard time even with a good candidate against Garrett, and why Republicans didn’t seriously contest NJ13 when Menendez left it. Unlike Congressional races, where often one candidate is hardly covered in the news and has hardly any campaign budget, the two party’s Presidential candidates are well known. The PVI index is widely used to identify competitive districts.  

Here’s Silver’s idea:

Are there any systematic differences in the ways that votes tend to fall for the Congress, as opposed to the Presidency? Are certain districts better or worse for Democrats, or Republicans, than PVI alone would suggest?

It turns out that there’s one other factor which is fairly useful to look at, which is socioeconomic status. Relative to how they do for the Presidency, Democrats are somewhat more likely to win races for Congress in poorer districts, and somewhat more likely to lose them in wealthier ones. Another way to put this is that a split ticket of Republican for President, Democrat for Congress is more likely to occur in a poor district, whereas a split ticket of Democrat for President, Republican for Congress is more likely to occur in a wealthy one.

Click through for the statistical analysis he uses. Silver expresses his PPI index as the chance for Democrats to win an open seat in an average election cycle, based solely on two factors: the recent Presidential Vote and the percentage of the population with incomes under $25,000/yr.  Here are the results for New Jersey:

District Name PVI PPI
NJ11 Frelinhguysen R+7 2.5%
NJ5 Garrett R+7 3.2%
NJ4 Smith R+6 10.9%
NJ7 Lance R+3 13.9%
NJ3 Adler R+1 27.9%
NJ12 Holt D+5 62.9%
NJ2 LoBiondo D+1 66.0%
NJ6 Pallone D+8 85.2%
NJ9 Rothman D+9 88.8%
NJ8 Pascrell D+10 96.6%
NJ1 Andrews D+12 97.0%
NJ13 Sires D+21 99.95%
NJ10 Payne D+33 99.998%

The main lesson, if you take this ratings seriously, is that New Jersey’s wealth makes the battleground Congressional districts lean Republican compared to how they vote at the Presidential level. In many states, the R+3 and even the R+7 districts have a great chance of going Democratic at the Congressional level, but here NJ5 and NJ7 are actually quite unfavorable, and should vote for the House like R+14 districts in the rest of the country. When we evaluate how our candidates did, it’s worth keeping this effect in mind.

Frank LoBiondo’s district is the poorest in New Jersey, and by this measure is slightly better for Democrats than Holt’s district, but we are stuck with the echo of 1994. In case it’s not obvious, being an incumbent matters, scandals matter, and cycles can be more or less Republican than the average cycle, and you should always remember that the most likely outcome doesn’t always happen. All of our 2010 races have incumbents so the percentages definitely do not apply. Also, this is the last election in the current districts.

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