A new study in the journal Social Science Research argues that a significant shift in GOP support for environmental concerns occurred after the fall of the Soviet Union.
The study is based on the League of Conservation Voter's annual report cards from 1970 through 2013 tracking politicians' positions on environmental issues. Mother Jones has a write-up on the findings with illuminating graphs clearly depicting a partisan split in both the Senate and House beginning in the early 1990s. Until then, while Republicans were still less concerned about the environment than Democrats, the two parties were much more closely aligned.
This makes sense when you realize some deniers have conflated environmental concern with Communism, like James Delingpole in his book, "Watermelons: The Green Movement's True Colors" (green on the outside, red on the inside).
Grist has a slightly different, much more controversial take—suggesting that the GOP started "hating green" when the environmental movement "started helping blacks" through regulations intended to reduce environmental impacts on minority and low income communities.
In either case, the study provides further evidence that Republican denial has less to do with the state of the science and more to do with political ideology.