While I have a healthy disrespect for polling since the 2016 General Election, I've got to think that resist Trump as a principle policy marker might not impress the voters that the Dems need to secure in the mid terms and the general.
I am glad you posted that chart. Each year is becoming less PC to say republican. I believe this PC crap picks off a few young people but also motivates a lot of those people who hate seeing their opinions supressed.
So my thesis is that a relatively large amount of don't know is likely republican voters if the party puts up an non establishment but acceptable candidate.