overnight layover: short-checked luggage or checked-through luggage?

You need to check with American Airlines then contact Iberia to confirm. Each airline and even each airport will have their own rules for checked baggage on this type of layover. Even with all of the answers in advance, be ready for an entirely different answer when you arrive at OPO.

Will somebody in Madrid send my bag to the baggage claim as soon as
they see there's no CLT flight until the next day?

I really doubt it. You're giving people too much credit for thinking.

If you want to collect your luggage in Madrid, you should tell them when you check it that you want to check it just for Madrid, then recheck it for Charlotte the next day.

This might be a good time to use a carryon just for what you will need for the layover.

I wonder if you could find answers to these questions on the American Airlines website? I have the same questions as you for an upcoming trip. Our flight legs home are DBV-CDG-ORD, with the layover at CDG being overnight (15 hours). When we check in at DBV, we want our bags checked all the way through to ORD so that we only have to take a day bag with us to the CDG-area hotel I booked, and so we won't have the hassle of re-checking luggage the next day. I've been wondering if it's up to (a) a set company policy, (b) my choice, or (c) the whim of the agent at DBV. Our flights were booked on Delta but both legs are operated by Air France. I went onto the AF website and found that "in most cases your baggage will be checked through to your final destination," but you need to collect it in these two specific cases: if your ongoing flight is from a different airport (e.g., fly into Charles de Gaulle but out of Orly) and if you are traveling with an animal in the hold (both the animal and baggage need to be picked up.) It also says this about a situation where your flight lands at either CDG or Orly and your onward flight is from the same airport: "If you wish to collect your baggage between these two flights, you will be charged €275, regardless of the number of bags, your travel cabin or the reason given." So on AF, company policy is checked-through to final, but there are two exceptions, and if the passenger WANTS to collect it, there's a hefty extra fee. See if AA has a place on its website where it discusses the rules for checked baggage.

Way too much discussion about a simple problem. Check your bags through to your final destination - Charlotte. This happens all the time and airlines know how to handle overnight luggage. I would have a carry on bag with my toothbrush and whatever else I need for an overnight stay. In the morning you will need to check in about three hours early, go through exit immigration and get on the international flight home. Really pretty easy and simple.

Way too much discussion about a simple problem.

It is not necessarily as simple as checking a bag through as some airlines do not allow that on overnight layovers. It looks like Iberia is one of them. Here is what the Iberia FAQ says

”but you will have to collect it and check it in again for your connecting flight if: … Your connecting flights are not on the same day and the total transfer time exceeds 12 hours.”

You can see it here. Scroll down to ”Checking in baggage on flights with stopovers”
https://www.iberia.com/us/faqs/checked-baggage/

My experience with American Airlines and a more than 12 hour layover at DFW (SFO-DFW-MAD flight) was that we could not check our luggage through to Madrid as the layover was more than 12 hours. We had to pick it up and recheck the next day for our DFW-MAD flight. Don’t know if the same protocol applies to other airlines or countries.

My situation is both similar and different from yours. Similar in that I flew this afternoon from Porto to Madrid, am spending the night in Madrid, and will fly to the US tomorrow….all on the same ticket. Different in that I flew on Air Europa (a Delta code share partner) then on Delta tomorrow. Also different in that I wanted my relatively small bag with me at my Madrid hotel. I made clear in Porto that I wanted the bag checked to Madrid only.

I’ll be interested in knowing whether American permits you to check your bag all the way through to CLT.

Will my checked luggage be tagged for MAD and claimed on arrival there?

Probably. Based on the Iberia policy ( found by Laura), their policy limits the transfer time to 12 hrs.
I spent 10+ minutes on the AA site and could not find a comparable statement about long layover bag policy. If it is on the AA website, it is not obvious.

Will a check-in counter agent in Porto have the final say on this?

YES

A 'simple' question, but the airlines make finding the answer difficult.

I think AA allows for a 24-hour connection, but in this case, I'd stick to Iberia's 12-hour guidance and not seek an exception. IB does all the ground handling for AA at MAD, and they may simply not be set up to handle an exception. Mabe a hassle, but better than losing the bag.