Census Shape Files
Boundary and Annexation Survey
The Census works with states ahead of the decennial census to ensure districts like precincts, municipalities, and school assignment aren't unnecessarily split by new census geography. To do that, states have to submit updated districts and alignment files. This is called the Boundary and Annexation Survey (BAS) and the Voting District (VTD) verification phase. Round 1 of the 2019 phase is underway and a good chunk of states/counties have submitted their work.