The charts above reflect information in a recent Gallup Poll. Gallup questioned 13,852 adults during 2018 to find out their ideological stance. The poll has a margin of error of only 1 point.
It shows the percentage who consider themselves to be conservative (35%), moderate (35%), or liberal (26%). Note that neither the conservatives nor the liberals have enough to win an election. Both would need the support of moderates.
The Republicans have hurt themselves among moderates by becoming too extreme -- and that was clearly shown in the last election (when voters gave Democrats a 9 point advantage over Republicans in votes on House races).
Some Democrats want their party to move far to the left. That would be a mistake. You don't appeal to the moderate voters by becoming extremist. If Democrats want to win in 2020, they must appeal to moderate voters. This does not mean a progressive candidate cannot win, but it must be one that appeals to a majority of moderates -- a candidate that moderates view as being mainstream.