The GOP talking point that any criticism of Sarah Palin is sexist reminds me of former Washington DC Mayor, Marion Barry. Barry at one time was a very promising African American politician. However, he was convicted of buying cocaine. Despite videotaped evidence, Barry repeatedly claimed that his conviction was due to racism, and responded to any criticisms regarding his administration or his conviction by invoking racism. In the end, the cries of racism did not prevent Barry from serving his sentence. Yet, these claims did prove to be very divisive.
Marion Barry did eventually comeback to claim local political office. However, any broader aspirations were killed by his conviction. Also, while his claims of racism were effective in energizing his base, it also generated some resentment that Barry was using racism to avoid being held accountable for his actions.
Here is a NY Times story on Mayor Barry:
http://query.nytimes.com/gst/fullpage.html?res=9D0CE2DD1131F934A15753C1A967958260