Consider the excellent {tigris} package. It interfaces directly to the US Census Bureau servers. Note however that the shapefiles can be detailed (and rather large as consequence).
For inspiration consider this code:
library(ggplot2)
library(tigris)
library(dplyr)
library(sf)
#download all the counties - incl. AK, HI
all_counties <- tigris::counties(year = 2015, class = "sf", resolution = "20m")
# lower 48 only
lower_48 <- all_counties %>%
filter(STATEFP < "60") %>% # territories out
filter(!STATEFP %in% c("02", "15")) # AK & HI out
# make a plot to check
ggplot() +
geom_sf(data = lower_48) +
coord_sf(crs = 5070) # Albers projection