Romney actually lost with highschool educated only and he lost significantly with postgraduate studies only getting 42% of the vote with Obama getting 55%.
So really the trend is that academics are liberal, not the highly educated.
(That 3% gap in college grads likely accounts for a larger percentage of the population than the massive 13% gap among grad degree holders)
http://ropercenter.cornell.edu/polls/us-elections/how-groups...
edit: another source that adds evidence to my argument
Professors in humanities that identify as Republican range from 6 to 11 percent.
Social Sciences 7 and 9 percent
2% of English professors are Republican.
Meanwhile 18% of social scientists are Marxists.
This evidence leads me to believe that Academics self select to be liberal and that people with a conservative mindset do not find academia appealing for whatever reason. The reason I rule it has little to do with intelligence of the groups is the prevalence of Marxism amongst academics. Intelligent people that use evidence to reason about their beliefs wouldn't be Marxist.
http://www.nytimes.com/2016/05/08/opinion/sunday/a-confessio...