American voters are broadly skeptical that President Joe Biden is mentally fit to serve a second term, and most do not think his potential general election rival Donald Trump is mentally fit either, according to a Quinnipiac University poll published Wednesday. Sixty-four percent of respondents in the poll said Biden was mentally unfit for another term, a bleak data point for the president in a survey where he nonetheless outperformed his topline numbers in many other polls. Fifty-one percent of voters told the pollster they did not think Trump was mentally fit for a second term on Pennsylvani…