Citric acid. It's the stuff they sell to clean coffee makers and similar appliances. It is also used in the food industry so you can sometimes (rarely) find the powder/crystals in the seasonings section at Walmart and elsewhere for cheaper than the coffee cleaning stuff. I always get a few pouches whenever I see it because it's not common but very useful for that purpose.

Throw a bucket of water directly in the toilet bowl so it will flush itself out and leave only a shallow water level, so less water in the bowl means more concentrated acid solution. Pour 2-3 table spoons of the stuff and leave it overnight. Next morning, use the toilet brush to finish off the job.