Yesterday I donated to Act Blue but today...

I'd say you buying regularly from Amazon is a good reason for them not to be suspicious about it. It's the unusual activities that get flagged.
But I also don't know whether stolen cards have been used for political donations.

the back of the credit card, not any number embedded in the email

That should eliminate the possibility that a third party was trying to get information from you

Most likely they just want to make sure you authorized the charge, especially if you hadn’t donated through act blue previously

👍

The answer is “no”, in my opinion.

If not, it's an irregular behavior that might trigger your credit card company's algorithm.