For a hackathon put together by the U.S. Census Bureau, Tableau, and data.world, I wanted to pull Census tract-level data for the whole country. While the Census API only allows data at this geographic level to be called for a single state at a time, someone put together a fantastic R library called ACS that can call data from the Census API, permitting data for the entire country to be called at once. The code I used to pull national data at the tract-level for my project is in this repo.