Yes, when "Philippic" refers to any of the speeches made by Demosthenes against Philip II of Macedon, it is often capitalised. Similarly, the term may be capitalised in other contexts to refer to a passionate invective or tirade, especially if it draws inspiration from or is reminiscent of Demosthenes' original Philippics. However, when used in a more general sense to describe any bitter attack or denunciation, the term can be written with a lowercase "p" as "philippic."