A prominent Democratic donor worried about the party's chances of winning the presidency emailed dozens of fans of Vice President Joe Biden on Friday, urging them to remain prepared to donate if Biden jumps into the race. The donor, Bill Bartmann, cited new polling showing Senator Bernie Sanders of Vermont nearly tied with the Hillary Clinton, eroding the 30-point lead the former secretary of state held at the end of last year. Bartmann and other party insiders are concerned that Sanders, a self-proclaimed Democratic socialist, is too far to the left to win against a Republican in the Nov. 8...