You're missing the core point. The politician doesn't need to say no to corporate money. He can, and should, say yes to all the money that any person or business wants to donate. The problem is when the politician writes into law a regulation, tariff, or rule that benefits a specific donor.
I fundraise for my kid's school. A local bank, ABC Savings, makes a considerable donation. There's no expectation that the school will pass a rule where we only accept students if their parents bank at ABC Savings. There's a difference between accepting campaign fundraising and quid pro quo.