20% of Dems don't support him.\
But no matter. There is no question that more Dems were polled than GOP, but it is not likely this was done on purpose.
This was going on last year too. Only Rasmussen has his telephone robot keep calling until he gets the desired partisan mix (with that desire equal to the turnout at some previous election). It's largely why he was right.
The other pollsters don't do this, but they are still random because it is in their financial interest to be random. It may be a change in the electorate that causes this. It may mean that more Dems want to respond than GOP. This would be consistent with what was seen in exit polling. GOP voters don't want to be bothered to entertain in the media. They prefer to speak only where it counts -- at the polls.
It is cheaper to poll 'national adults' rather than 'likely voters'.
It is biased to undersample Republicans and oversample Democrats.