While the nomination could be well within his reach, a presidential victory over an incumbent president could prove more difficult.
Democratic Vice Presidents, excluding LBJ, who was president at the time, Democratic sitting and former Vice Presidents during my lifetime have been unable to win a presidential election.
Why? Well, two of the three, since Biden has yet to get the nomination, were nominated after overcoming serious opposition. Democrats love fresh faces. Jimmy Carter, Bill Clinton, and Barak Obama were barely known in the years before winning the White House. To that end, they picked established names out of the senate. These were solid long time Senators. Joe Biden would certainly fall in that category. They may have used party connections to beat out other candidates who could generate excitement (Eugene McCarthy, Gary Hart, Bernie Sanders), with the exception of Gore, who had to fight a 3rd party effort by consumer advocate Ralph Nader in the general election cycle, getting rank and file support proved to be difficult.
