First, download the Texas population by county here. Download it as a *.csv file and remove the line giving the results for the entire state.

texas.pop <- read.csv("2011_txpopest_county.csv", header = TRUE)
texas.pop$census_2010_count.pcnt <- texas.pop$census_2010_count/sum(texas.pop$census_2010_count) * 
    100

Second, download the longitude and latitude data here

library(maptools)
usa.dist <- readShapeSpatial("USA_adm2")
texas.dist <- usa.dist[usa.dist$NAME_1 == "Texas", ]

Third, plot the map

library(ggplot2)
library(scales)
texas.fort <- fortify(texas.dist, region = "NAME_2")


ggplot() + geom_map(data = texas.pop, aes(map_id = county, fill = census_2010_count.pcnt), 
    map = texas.fort) + expand_limits(x = texas.fort$long, y = texas.fort$lat) + 
    labs(title = "Percentage of 2010 Texas Population") + scale_fill_gradient2(space = "Lab", 
    name = "Population %", limits = c(0, 17), midpoint = 4, high = "white", 
    mid = muted("darkgreen"), low = "white")