Joe Biden and Donald Trump Ron Adar | Echoes Wire | Barcroft Media via Getty Images; Mandel Ngan | AFP | Getty Images Most voters in six 2020 swing states do not consider either President Donald Trump or Democratic presidential nominee Joe Biden mentally fit to be president, according to a new CNBC/Change Research poll.    The findings in the pivotal states of Arizona, Florida, Michigan, North Carolina, Pennsylvania and Wisconsin were released Thursday amid a sustained push by the Trump campaign to portray Biden as incapable of handling the demands of office. Most respondents in those states, which will play a huge role in determining the presidential election winner, do not have confidence in Trump's ability to take on the job, either.  A 51% majority said Trump is mentally unfit to be president, while 49% answered that he is fit to hold the job. A similar share, by a 52% to 48% margin, responded that Biden is mentally unfit to be president.