Questions of mental fitness: Fair game or dirty politics?

Joe Biden on Tuesday bristled when asked by a reporter whether he would take a cognitive assessment test similar to the one President Trump has occasionally boasted about passing. “Why the hell would I take a test?” the presumptive Democratic presidential nominee said.

The question about a cognitive test, which is designed to identify conditions like dementia, is the latest instance of the concerns of mental fitness being pushed to the center of the presidential race. Trump, 74, has repeatedly claimed that Biden, 77, is experiencing intellectual decline. His campaign...