In the dictionary, urban is defined as pertaining to characteristics to cities. As far as how it is defined by the census, the defined "urban areas" sometimes have large coverage and can include some places that are more suburban. Bakersfield is a good example of Republican voting urban area, since it has fairly dense pockets of Republican strength and urban areas are defined a lot by density. Density does count, which explains why there are few urban areas in the plains, which is not very dense at all.
Until 1940-ish, the Census Bureau defined "urban" as any municipality with more than 2500 persons. They then started adding in adjacent territory that was higher density.
Since 2000, the core of an urban area is simply a densely built up area with a density of more than 1000 persons per square mile, along with surrounding areas with a density of 500 persons per square mile, that has a total population greater than 2500.
Pampa, TX was pretty close to 90% for Romney (Gray County was 87.2% for Romney), and Pampa is 81% of the county population.
