A data.frame which contains a map of all 50 US States plus the District of Columbia. The shapefile was modified using QGIS in order to 1) remove Puerto Rico 2) remove islands off of Alaska that crossed the antimeridian 3) renamed the county "Dona Ana" (which is properly written with a tilde over the first "n") to "Dona Ana" because R CMD check emits a warning if data contains non-ASCII characters 4) some columns were added for convenience.
Taken from the US Census 2010 Cartographic Boundary shapefiles page (https://www.census.gov/geo/maps-data/data/tiger-cart-boundary.html) in May 2014. The resolutions is 20m (20m = 1:20,000,000).
# render the map with ggplot2 library(ggplot2) data(county.map) ggplot(county.map, aes(long, lat, group=group)) + geom_polygon()