Because global warming leads to having to do something about it. Some are not keen on doing anything as long as they are shareholders in one of the things causing global warming.

Those believing in global warming are also those who have a tendency to blame their governments and big business for all the evils of the world. Those who don't believe in global warming tend to believe that the media over-sensationalizes everything to make more money. Guess which is the left wing and which is the right wing.

Because republicans get a great amount of their votes from big business. Big business also does not like any sort of restrictions that affect profit. A politician must stay loyal to their voters so you can see the trend develop.

All of the other responses have correct points in them. I'd like to add that people who do not believe that humans are responsible for some climate change do not have to accept personal responsibility thus need make no changes to their lifestyles. I know personally that Republicans and Democrats alike believe in Global Warming, and then there are people from both parties who do not believe. It isn't JUST politically motivated; it is motivated often by self interest. If you believe in Global Warming, you might just have to reconsider your habits - something many humans just don't like to do. Change is not a welcome thing to them. If you believe that the planet can not sustain the numbers of humans on it, then you would have to practice birth control and that is frowned on in most religions. If you believe in Global Warming, you might just feel REAL fear rather than a construct of fear which is what our politicians are so fond of feeding to us. It is easier to believe doom and gloom predictions about the economy than have to face REAL doom and gloom (not to say economic issues are not important, they are, but again, most economic factors are resistant to change as well). The status quo pays dearly to keep people believing what is expeditious to them.