I think what happened is when the GOP seem to abandon the Roosevelt platform after WW2, in favor of more Reaganite types, some of TRs and FDRs supporters shifted to the Dems or simply stopped voting GOP.
The Dems took over the environmentally-minded voters, and the GOP started failing/not undertaking PR outreach to them. Then Korea and Vietnam happened, and the GOP ended up smeared as 'warhawks' as well.
Trump is really the first GOP pres in a while to actively work on reaching out to environmentally focused voters, and his permanent funding of the Nat'l Park Service is one of his underrated achievements.