A recent research article by Andrew Hall entitled “What Happens When Extremists Win Primaries?” asked whether parties who nominate more extreme candidates in a primary face a penalty in the general election. To test this question, he examined U.S. House elections from 1980 to 2010 and found that the party that nominates a more ideologically extreme candidate over an ideological moderate tends to lose, on average, somewhere between 9%-13% of the vote in the general election. This reduces the chance of winning the seat by anywhere from 35%-54%. The take-away from this research article is that parties stand to benefit from nominating more moderate candidates and take big risks when they nominate more ideologically extreme candidates.
Political scientists often argue that partisan and economic “fundamentals” matter more than campaign events or candidate characteristics when it comes to predicting the outcome of political elections. Thus, whether Republicans nominate Jeb Bush or Rand Paul, it will end up mattering only a little in terms of the final outcome of the election. This effect, however, is most strongly the case when it comes to nation-wide presidential elections. The further one goes “down the ballot,” the less the “fundamentals” tend to matter and the more campaign events and candidate characteristics come into play.
In the case of gubernatorial elections, I would argue that they’re roughly equivalent to U.S. House congressional elections in terms of the relative effect of “fundamentals” vs. campaign events and candidate characteristics. Thus, I don’t think it unreasonable to assume that, all other things being equal, the effect that Professor Hall found for U.S. House elections would generally apply to state-wide gubernatorial elections as well.
This suggests that the Kentucky Republican party did themselves no favors by nominating Tea Party ideologue Matt Bevin over establishment Republican James Comer on in the May 19th Republican gubernatorial primary. It may have potentially cost them up to 10% of the two-party vote come November, giving a respectable advantage to Democratic candidate Jack Conway.