Here is something to consider. If a student and a teacher has opposing political views no matter how good the arguments will it be unfair of the teacher to downgrade the student on a political basis? How can you tell if that was the reason if the teacher argues that it was based on a lack of evidence?
That is a tough question to answer. Yes it would be highly unfair to give a student a bad grade because of his or her political view. The fact that it would be hard to prove is the big thing here.
I don't call for people to get fired often, but such a scenario would provoke such a response. The only time politics should enter a classroom is if it is the subject of study, and then it must be presented as neutrally as possible. For it to impact a student in such a way would be a massive offense, easily on par with a teacher taking bribes for good grades
Now, with that stated, Kntoran is correct, in many cases, proving the reason for the grades is related to political differences rather than understanding will often be difficult.