I checked for plugged drainage tubes, but all were clear, so I concluded the pump was bad and ordered a new one. That solved the problem.
I removed the front of the washing machine by opening the two top catches with a putty knife. I used channel lock pliers to move the spring clamps away from the pump on the inlet and outlet tubes, and a 3/8 inch socket wrench to remove the pump mounting screws. Only three common household tools were required. The machine works like new again.