I poked around on the internet, and the most likely cause was described in multiple places as a clogged or failing pump. Following various videos, I first tried clearing out the filter, with no success. Running a couple more tests, I observed that the pump was (noisily) running, but no water was being pumped. I took a leap and ordered a new pump. I also ordered a new pump output clamp, because mine was a crimp-type single-use clamp (be sure to check yours) that I had to remove by use of a pair of end-nippers. I did a search of "removing crimp style hose clamps" to see how to do this without damaging the hose. Finding out that I would need this part was definitely a good learning from removing the old pump before I ordered the new one.
When the new pump arrived, it took me probably 20 minutes to install the new one. Like it shows in some videos, connect the power pug to the pump before installing the pump; the new pumps are different from the old one in a way that it's more difficult to do this later. The hardest part of the installation was sliding it across far enough to push down the rubber locking tab. The rubber is new, and so it is much stiffer than the one on the old pump. I poured in 10 quarts of water with a bucket and started it up. The problem was solved.