Led by the oil and gas industry, this sector regularly pumps the vast majority of its campaign contributions into Republican coffers. Even as other traditionally GOP-inclined industries have shifted somewhat to the left, this sector has remained rock-solid red.

Since the 1990 election cycle, interests from this sector have contributed more than two-thirds of its campaign contributions to Republican candidates. Another big contributor in this sector - and another big GOP supporter - is the electric utilities industry. Less generous, but even more partisan, is the mining industry. [Read more Background]