"Archival research class"? Are you for real, Richmal? Have you read the thread on the TT historian with a well-cited book who can't get a spousal offer because he doesn't score any diversity points?
Junior-Sr year summer, pre-Ph.D.
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OP: I double majored in data science, so I have a firm Quan background with multiple languages (R, Python, Stata, SQL, C, C++) but I don't know a language. Coming from an Ivy where language is not required.
Polish, polish, and polish your best solo writing sample. One great writing sample is better than 2, 3, 4, 5 very good ones. Published doesn't mean great or important. Co-authoring a published paper with faculty as an UG doesn't mean much. You only need one good, solid, important topic to finish your PhD, and that will be your dissertation topic.
Focus on originality and creativity in your work. Many people can master the canon of their fields to teach it and pass exams, but what leads to true success is originality. This is hard to cultivate; in-born talent comes into play here.
Work on impressing profs to get great LOR and make sure they know you, your best work--that one best paper, and your interest in poli sci well-enough, and that they like you, to write substantive, detailed LORs. If you're not likeable, you're in trouble.
Take a philosophy of science class. Take an upper-level theory class. Become more well-rounded intellectually.