This poll, like the one from CBS/NYT over samples democrats by 8%. One of the comments on Conservative Treehouse said when adjusted, Trump leads Crooked Hillary 52% to 45%.
But 8% more Americans are Democrat affiliated or leaning:
http://www.gallup.com/poll/15370/party-affiliation.aspx
An even polling would not be representative.
I think the over-sample is a discreet way to account for the fraud factor. This result signals that the Dems will have to exhume a few million more dead voters for them to have a chance. A tall order, even for them.
Exactly what makes you think that the voters in November will split exactly 50% between republicans and democrats. It has never happened and won't this year, either. Trump can and will win democrat votes, but more democrats will come to the polls than republicans. That is the easiest stat to predict for this November.