In trying to diagnose a little more, I took the brushes off just to see how worn they are. They are definitely worn down a bit. I'm going to order replacements. But after putting the motor back together, the mower seems to have a little more power. Not as much as it should, but noticeably more than it did when I posted last week.
I also checked the battery voltage a little more closely after reading in several places that the batteries should read higher than 12V at rest. The upper battery looks to be right about 12v on the nose (with an analog meter). The lower battery appears to read a bit higher than 12V. If the new brushes don't solve the problem, I may buy one new battery first. Anything wrong with not replacing them as a pair?