The Democrats' 2020 dilemma: Rally base or persuade moderates?

Richard Nixon, who tied Franklin Roosevelt’s record by running on a national presidential ticket five times, said that he ran to the right to win the Republican nomination but then ran back toward the center in the general election. In the 2004 presidential campaign, however, President George W. Bush and Karl Rove found a new path to electoral victory by turning out their base rather than attracting the undecided.

Now, Democrats face this dilemma in 2020: Should they nominate a candidate who is further to the left in order to energize their base (think Bernie Sanders, Elizabeth Warren, or Kamala Harris), or should they instead steer center-left with Joe Biden?