Oh, I thought you were being sarcastic. It was clearly Citizens United (2008) and the explosive growth of Super PACs that followed.

Unlimited spending from the fossil fuel industry basically standardized Republican candidates on climate denial talking points. Plus whatever bizarre fetishes random Republican billionaires had, like Adelson keeping Gingrich's primary campaign on life support for months. That fucked up Romney's pivot to the general election for no perceivable gain to any of them. According to Wikipedia Adelson spent over $90 million on losing candidates in 2012!

All of that was through Super PACs.