Which interest groups have the greatest influence on the policymaking process in Texas?
Texas is obviously a very large state, but the issues that drive it, and for which special interest groups are most prominent, are oil, education, guns, states' rights, and agriculture. This is not to say that these are the only, or even the most important issues (polls of Texans show the economy, education, and water policy are the most important issues) to most Texans, but, given the scope of the question, these are the ones that would draw the most attention from special interest groups within the state of Texas.
Oil, as pretty much everyone knows, is one of the state's most important sources of revenue and is also an important source of jobs. Taxes paid by the oil and gas industry in Texas is a vital source of funding for state schools, social welfare programs, police and fire departments, and more. Consequently, interest groups associated with the oil and gas industries are always busy working with Texas state legislators to ensure that energy industry interests are protected.
Texas is also a very independent, and generally conservative state. To the extent that individual states pass their own gun restrictions, anti-gun control interest groups are active in lobbying state legislators to ensure that gun rights are not infringed upon by local government -- not something likely to happen in the near future anyway.
Agriculture is also very important to Texas, politically, economically, and culturally. Interest groups representing various aspects of the agriculture and ranching industries play a big role in lobbying on behalf of those industries, especially with access to water becoming a more pressing issue in the Southwest United States.
Texans are fiercely proud of their heritage. They see themselves as the heirs to a tradition of independence, revelling in the knowledge that the state's founders fought the Mexican government on their own. "Remember the Alamo" remains an important rallying cry in Texas. As such, issues of states' rights remain extremely important there, and any effort by the federal government that is seen as threatening to that independence generates a lot of political activism by groups interested in maintaining the image of an independent, proud state willing to go its own way.
Texans do not like being told what to do by the federal government. They are in no way disloyal to the United States; on the contrary, they are proud Americans. They hold fast, however, to the notion of Texas as a distinct, independent entity.