Stupid question: Doesn't a (social or legal) norm to return money donated by crooks make politicians more willing to sweep their donors' misdeeds under the rug (and push judges to do the same), in order to avoid having to justify themselves?

Possibly, but the risk appears substantial for a relatively modest payoff.

If their guilt is glaringly apparent and your stance is interpreted as supportive, the blowback could be even more detrimental compared to promptly creating distance and professing unawareness of their wrongdoing, genuine or not.

The level of risk seems considerably high for an amount of money that, in the realm of campaign contributions, isn't substantial.

Worse, if the money is dirty, it might give prosecutors a reason to start investigating your campaign. Many federal prosecutors are more than eager to make their name in the Justice Department by taking down a prominent public figure.