Asking a professor to change a grade-- GPA issue

<p>This semester i took a philosophy class courses. I have As and A+ in all my assignments but in midterm i took a C. My issues is that i want my GPA average to be above 3.5 and for that i need at least an A- in this class. All my assignments, the midterm and the final exam are graded by the TA. Since the TA is the one who basically decides my final grade, i want to contact her before the grades are posted that i need to get an A- for this course, because i believe that if i ask to change my grade after its is posted, probably it would not happen. The issues is that i want to make sure that i take A- in, although i don't know what is my grade for the final exam. i really don't know how to approach her.
Should i contact her before or after my grade is posted?
Or, should i directly contact the professor? How should i present my issue to her?
If any one has any experience with a class where although the professor is the one who posts the grades, the TA is the one who decides our final grade, advices are welcomed. This is the first time that i find my self in this situation and i really don't know what to do.