CNBC/Change Research asked 4,143 likely voters in Arizona, Florida, Michigan, North Carolina, Pennsylvania, and Wisconsin what they thought of the two major party candidates this year. “A 51 percent majority said Trump is mentally unfit to be president, while 49 percent answered that he is fit to hold the job. A similar share, by a 52 percent to 48 percent margin, responded that Biden is mentally unfit to be president.”
The same poll found Biden ahead, 49 percent to 45 percent, cumulatively across those six states. If only 48 percent think Biden is mentally fit for president, and he received 49 percent in the same sample, it means there are Biden voters out there who think that their man is mentally unfit to be president, but are voting for him anyway.