The chart above reflects the results of a recent Gallup Poll -- done between January 16th and 29th of a national sample of 1,033 adults, with a 4 point margin of error.
It is just a fact that there are not enough Democrats (or Republicans) to win a presidential election by themselves. The eventual winner of the presidential election will be the candidate who convinces the most Independents (most of whom are moderates) to vote for them.
That presents a problem for Democrats. Democrats aren't scared by the word "socialist", and in fact, a candidate who calls himself a socialist (Bernie Sanders) is doing very well in their primary. But the problem is that Independents don't think the same way. Only about 45% (less than half) of them say they would consider voting for a socialist.
That's not enough. If a majority of Independents vote Republican (or stay at home), then Trump could be elected to a second term.
Democrats say they want a candidate who can beat Trump. Bernie Sanders is not that candidate.