Please feel free to correct the title, I am not sure how to formulate this crisp & clear in a sentence.
I am very interested in a particular graduate program. It's a top notch program, but I happen to want to work with an academic who is quite young still and not yet a professor, and although the program is o.k. for the subfield I am interested in, it is not stellar. (Let's not discuss here whether this would be a clever move please)
Question: Will it affect my chances getting into the program, that is, would a university rather see students interested in the field they are already strong in, or would they like to see someone being interested in something for which the uni is not particularly known for?
Will the prospective advisor maybe have less of a say regarding my admission, than if I were to target a senior professor who would like to work with me?
There is no way to know. Some departments may rank candidates independent of their desired specialty while other departments may weight who the applicant wants to work with (and who wants to work with the applicant) heavily. For departments who give weight to the sub-field, the department might be trying to strengthen a weakness or only accept students in areas they are strong.
I am not really sure why the departmental politics matters. Trying to deceive (in a mild way) the admissions committee is not likely going to be advantageous. Your strongest application is going to be to talk about the area you are most excited about. Further, if you get accepted based on a falsely expressed interest in A, you may not be able to switch to B later.