Former Vice President Joseph R. Biden Jr. is in the final stages of deciding whether to run for president and has told allies he is skeptical the other Democrats eyeing the White House can defeat President Trump, an assessment that foreshadows a clash between the veteran Washington insider and the more liberal and fresh-faced contenders for the party's 2020 nomination. Many Democratic voters, and nearly all major Democratic donors, are keenly interested in Mr. Biden’s plans because of their consuming focus on finding a candidate who can beat a president they believe represents a threat to American democracy. But there...