[borism]

The corporations control both fiscal (tax revenue) and political (jobs) levers of the legislature. The politicians in the US (or most democratic countries for that matter) cannot simply say no to the money from large multinationals. Especially in the US where any interference with the business e.g. nationalisation is considered communist taboo topic.

[davetufts]

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.

[borism]

I don't see how (s)he can be saying yes to all the money coming in and NOT be favoring one over another. It's not like everyone is giving equal amounts of money?

In your own example - it's not like school will not accept somebody if they're not using ABC Savings. But they sure as hell will cater more to those who do, than to those who don't?