I doubt it will be +8 on election day since I think the democrats will have a depressed turnout if I believe what I am reading in the paper since many democrats think the superdelegates are stealing the election from Bernie.
Probably the more important thing is to look at the trend from now to November. We should expect that democrat advantage to gradually disappear.
But, I am realistic and well aware of 2012 that Romney had a new data driven get out the vote machine and it didn't work. Romney was so convinced he was going to win that he had no one prepare a concession speech on election night. Obama knew how to get out his supporters. Black vote made up 13% of the vote - a record. The black vote over represented their population for the first time in this country and no republican thought Obama could duplicate the turnout of 2008. Obama did better. He beat Romney in the ground game. Polls are one thing, but your voters have to get to the polls.
I'm a bit concerned that Trump is going to let the republicans worry with the ground game based on his disinterest of that in the primaries. We will see.
No, I expect polls to continue to oversample Democrats. A random phone poll will always do that. Some pollsters will adjust the results, but others won't.
Obama did better. He beat Romney in the ground game. Polls are one thing, but your voters have to get to the polls.
Yes, Obama did very well: he had a higher voter turnout in some districts than there were known to be voting-age adults.