Is bipolar disorder a problem with chemicals in the brain, or a mental illness?

There is a movement to reclassify bipolar as a problem with brain chemicals. I wonder if this makes any difference in terms of treatment, or if it is mainly a political thing.

How could you tell the difference between a mental illness, and an illness that affects the brain in a less visible way (than, say, cancer). What difference does it make if we reclassify it? If we experience bipolar as it impacts our thoughts, how could it not be a mental illness? Like a form of dementia.