People do care; the way for leftists (like myself) to show that they care is by simply abstaining or going third party which would obviously cripple her.
Most Clinton voters are voting for her, not against Trump.
https://wikileaks.org/podesta-emails/emailid/26551
If this is real, it can be duplicated.
1.96sqrt(0.56*(1-0.56)/871) ~ 3.3%
FiveThirtyEight is extremely biased however, and greatly oversamples Democrats.
oversampling is done to get a better view of specific groups. when put back in comparison to the population as a whole they are readjusted for their extra sampling.
those polls were also for internal purposes to the clinton campaign, and not polls that are actually released. so why would there be manipulation in an internal poll that would affect decision making in the clinton camp?
there is literally zero going on in that email but trump supporters saw "oversampled" and freaked out because they didn't know what it meant in regards to polling.
edit: referring to the claim that Clinton voters aren't voting for clinton