The phrase "harps up on" could be replaced with a variety of synonyms, depending on the context and desired tone. For instance, the phrase "dwells on" could convey a sense of fixation or over-emphasis, while "elaborates on" could suggest a more detailed or in-depth exploration. Similarly, "hones in on" conveys a sense of precision or focus, while "fixates on" suggests an obsessive focus. Other potential synonyms might include "fixes upon," "picks at," or "obsesses over," each with its own nuances. Ultimately, the most effective synonym will depend on the writer's intended meaning and the context in which the phrase is used.