If the person hesitates in any way, look elsewhere.
Alternatively, you will be asked to submit two or three individual letters of recommendation, of which at least one or two must come from senior science faculty.
A letter written by a teaching assistant usually carries less weight.
Go to office hours; become a teaching assistant; volunteer to work in their lab; take them to lunch!
Whatever it takes so that when the time comes, they will be able to write you a personal letter of recommendation.