NASHVILLE, Tenn. — President Donald Trump and Democratic challenger Joe Biden offered sharply different visions of how to handle the surging pandemic and fought over how much Trump pays in taxes during their final debate of a tumultuous campaign. Final debates often play an outsized role in electoral outcomes but Thursday night’s showdown was different from those past, with more than 46 million votes already cast and fewer undecided voters remaining than at this point in previous election years.