The Balcones Escarpment is a bulwark of cracked and weathered rock that crosses Texas diagonally from the Dallas–Fort Worth Metroplex southwest to Austin and San Antonio and all the way to the Rio Grande. It separates the flatlands of central Texas from the stony hills to the north and west. It is a boundary between cropland and grazing land, between acres rich with greenery and acres whose rolling brown hills blaze out in color when the wildflowers bloom in Texas’s early spring, between places where the sky is hemmed in by trees and buildings and places where the sky seems to surround you. The Balcones Escarpment separates Dallas and Fort Worth; it runs through Austin and the western edge of San Antonio. But it is less familiar to Texans today than the highway that runs pretty much along the same line: Interstate 35. This is one of the most heavily traveled and congested interstates in America, thick with truck traffic in the populated stretches between the Metroplex and the Mexican border even as it passes through the lightly populated near-desert between San Antonio and Laredo. It is one of the great routes of commerce in America, or rather between the United States and Mexico.

I-35 connects Austin and San Antonio, two booming Texan cities with very different beginnings and different characters now. Austin is the creation of state government, with the pink marble Capitol and the sprawling University of Texas. But it has gone beyond its roots, becoming one of America’s leading high-technology and entrepreneurial centers, with an office-building boom downtown and in the suburbs. San Antonio was the creation of Texas’s Mexican settlers, a town with a Spanish accent and a heavily Latino population. It is proud of the Alamo and the Riverwalk, but it also has corporate headquarters, numerous lakes for water supply and recreation, and an array of military bases.

In the counties between these two cities and in the Hill Country to the west, is the Texas German country, originally settled by Germans fleeing the reaction against the failed revolutions of 1848. The Texas German country has always been a set of orderly communities in rip-roaring Texas, economically prosperous in a state that considered itself poor until it struck oil. It was anti-slavery and politically Republican in a state whose enthusiasm for the Democratic Party had roots in Confederate loyalties and populist rebellions. The Texas German heritage is still visible, and an antique German is sometimes heard on the streets in towns like New Braunfels, Boerne and Fredericksburg (there are about 10,000 speakers now, compared to 159,000 in the 1940s). These communities, with their neat houses, low cost of living and Hill Country ambience, attract new residents to new subdivisions and lakeside developments.

The 21st Congressional District of Texas includes much of this territory. About half of its people are in San Antonio and Bexar County. It includes the northeast corner of the city and county, Fort Sam Houston plus some north-side neighborhoods, with many houses being bought by rich Mexicans from Monterrey. This is mostly Anglo San Antonio, though 27% of the Bexar County residents in the district are Hispanic. About one-fourth of the residents are in Austin and Travis County, including the downtown University of Texas campus. This is the most Republican part of a Democratic county. The district includes all of New Braunfels and Comal County just northeast of San Antonio as well as several Hill Country counties to the west: Blanco County, where Lyndon Johnson was born, in Johnson City, and which was his legal residence when he was first elected to the House in 1937 (but only a sliver of the LBJ Ranch near Fredericksburg, just to the west); Kendall County, a fast-growing area north of San Antonio; Kerr County, the most populous part of the Hill Country, and two smaller counties to the south. The political heritage of the district is mixed. While Travis County was always Democratic and the Texas German country was Republican, San Antonio was mixed. Overall, the district is heavily Republican. It voted 59% for Republican presidential nominee John McCain in 2008.