After trying to find similar GeoJSON versions of US State and County boundaries, I finally decided to roll my own from the United States [Census Cartographic Boundary Files][1], converting them to GeoJSON and KML formats using the [MyGeoData vector converter][2].

I've made the results of this conversion available at: https://eric.clst.org/tech/usgeojson/. Feel free to use these if they will help.


  [1]: https://www.census.gov/geographies/mapping-files/time-series/geo/cartographic-boundary.html
  [2]: https://mygeodata.cloud/converter/