It's not even really the donors, per se, but their voters. Climate change denialism is very popular. The businesses ensure that candidates who favor them connect with those voters, but it's not like the candidate would suddenly change their mind if those donations dried up. They'd continue to be denialists. And if that politician leaves, the denialist voters will be sure to pick up another denialist candidate.
The business help ensure denialism not with the politicians, but by funding denialist news networks and web sites. They also run attack ads (on any subject, not just climate) to defeat candidates who would oppose denialism.
They don't need to buy politicians. They buy voters instead, by scaring them. You won't fix the candidates, who are just doing what their constituents (at least, 50%+1 of them) want. The direct donations are a pittance. It's the overall miasma of denialism that give us anti-intellectual politicians, not the other way around.
I've got no idea how to fix it. It's famously said that you can't fix stupid, and there's a LOT of stupid.