What is considered an unethical contribution to a Presidential campaign fund?

I know nothing about this, but was going through who gives $ to which candidate, and something looks funny to me.What is considered an unethical contribution to a Presidential campaign fund?
If you contribute to get access or favors then it's unethical.What is considered an unethical contribution to a Presidential campaign fund?
donating money to a politician or political party for their campaign is considered a form of speech. Therefore there is a level of strict scrutiny involved, which means that the for the government to impose a burden on the ability to donate money, they must show a compelling reason to support a government interest for any type of enforcement of an ethics violation