This is an incorrect framing of the situation. You aren't being asked for a Yes/No vote on Democrats. You are being asked if you prefer Democrats or Republicans. Or for this election, if you prefer Democracy or Fascism. If you vote "no preference", that does not communicate "I prefer the Democrats, but want them to move further left", either logically or politically.

There are lots of ways to communicate desired policy changes: letter-writing, primaries (including campaigning/funding for candidates), protests, marches, press, social-media, etc. Voting against your interest is not one of them.