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 FilesCensus Cartographic Boundary Files, converting them to GeoJSON and KML formats using the MyGeoData vector converterMyGeoData vector converter.
I've made the results of this conversion available at: http://eric.clst.org/Stuff/USGeoJSONhttps://eric.clst.org/tech/usgeojson/. Feel free to use these if they will help.