(For US Federal election law only) If someone were to give me a sum of money, and direct me to donate it to congressmen that voted a certain way, would I be able to donate it on their behalf (i.e. in their name)? Would I need power of attorney to do so? Or could I otherwise get authorization to act as an agent?
Campaign contribution laws require that the donor of campaign funds be disclosed and in some cases with dollar caps on contributions per person.
Now, if you are actually donating their money in their name, you really aren't doing anything different than a bank is in taking money of the account of a checking account owner and giving it to the recipient.
I don't see an obvious reason that you would have to have a power of attorney in writing to have the authority to do so, which could probably be granted orally, but you might still want something in writing documenting the transaction so that you wouldn't be wrongfully convicted of making a donation of your own money in someone else's name or at your direction rather than the direction of the true donor, which would be illegal.
It isn't clear, however, why someone would want to set up the transaction that way.