The Republican crossover theory debunked. In addition to many smart comments from 538 readers to the previous post on the SC race, I received an email from one particularly astute reader named Harrison Brown. Complete with an excel spreadsheet to back up his conclusions, Brown basically argues that there's neither any logic to, nor statistical evidence to support, the idea of Republicans crossing over to infiltrate the Democratic primary. Here are the key sections from his email to me, verbatim:
1. Suppose people were being brought into the Democratic-primary voting pool (from unregistered voters, the Republican faithful, or wherever) for the sole purpose of voting for Greene. Imagine a variable encapsulating the proportion of primary voters in each county who are Greene partisans; this (hidden) variable ought to be strongly positively correlated with both Greene's final results and with the participation rate in each county. In particular, this implies that Greene's vote share and the participation rate, both of which we can measure, would be correlated. But this is not the case -- under either linear or rank correlation! The R-squared and rho-squared are both effectively 0.
2. Even if that effect didn't show up, there should still be other signs. For instance, we can see if there are any counties where turnout for the Democratic primary exceeded the number of votes Barack Obama received in 2008; those would be prime suspects for Republican influence. And, in fact, there are three such counties: Hampton, Lee, and Union. But these are all fairly small counties where McCain/Palin received under 30% of the vote -- hardly Republican-dominated...
A more robust analysis of turnout levels reveals similar patterns. Although I didn't collect data for Republican voters (except for the McCain vote share), I came up with a rough estimate of GOP voters in 2008 by assuming the two-party share was 100% in each county. Running a linear regression to predict the number of Democratic primary voters from the number of votes Obama and McCain received, we find that the McCain raw vote total is statistically significant--but it has a negative coefficient. If anything, this points to voter suppression (no real surprises) rather than ballot box stuffing.
3. Finally, there's the simple question of where the Republican voters would have come from! From eyeballing the GOP primary totals, it seems like turnout in that elections was almost ludicrously high, which seems more-or-less corroborated by what Google's told me. But barring widespread voter fraud and/or corruption by local election officials, high turnout in the GOP primary should be incompatible with infiltration into the Democratic primary.
In conclusion, while the voting patterns in the D-Senate primary are strange and may not be totally legitimate, they don't bear the expected hallmarks that would arise in the case of a Republican plant.
http://www.fivethirtyeight.com/2010/06/sc-democratic-primary-getting-weirder.html