The correct answer to fill in the blank is:
El Paso, Mcallen, and Brownsville, has remained politically a Democratic Party bastion.
Along with the Republican Party, the Democratic Party is one of the 2 major political parties in the United States. The main difference between the 2 is that the Democratic party believe that the federal government must take an active role in people's lives, mainly those who are in need while the Republican party believes otherwise.