

Simple: The quality (for lack of a better phrase) of the republic partisan field of potential challengers. Case in point: Deranged right-winger GOP candidate Michele Bachmann. From the past weekend's headlines: "Michele Bachmann Promises to Ban Pornography, Gay Marriage." Good to see she really has her finger on the pulse of what's important to the nation these days. BTW, look up "unelectable" in the dictionary.
http://www.dailymail.co.uk/news/article-2012798/Michele-Bachmann-GOP-hopeful-sign-pledge-banning-gay-marriage.html