He may not be on the Democratic ballot, but the 2020 presidential candidate wielding the most influence over the party's remarkably unsettled primary fight is President Donald Trump.As the race for the next occupant of the White House enters the final weeks of 2019, awash in a surprising degree of uncertainty, Democratic voters have Trump on their minds as much as the crowded field of contenders jockeying for the right to challenge him. From a distance, Trump is hanging over every aspect of the Democratic contest.