[Joe Brower]

Over the last couple of decades, I've noticed that popular polls are often found to be weighted 15-20% against the GOP candidate when the votes are finally counted.

I interpret this as Trump having a good lead over Hillary at this time.

[crusher2013]

You are correct.

At the beginning of the campaign they are push polls designed to help discourage Republicans.

Near the election they tend to get accurate because they want to retain their reputation. (Knowing that people will forget the earlier polls).