My theory is if you hold partisan percentages constant, you automatically undo unintentional things that might skew results by party -- like perhaps polling on days or hours favorable to one side or the other . . . in terms of whether or not they are home to answer the phone. This also, btw, addresses selection of zipcode. Holding a mix constant is a valid methodology for a constant registration mix in the population.
But what it won't do is spot some surge in registration. Holding partisan mix constant would intentionally ignore a surge in registration.
Rasmussen holds mix constant and he's showing very little change all year long. He'll miss a registration surge, if there is one.
Those allowing mix to float . . . well, they will catch such a surge. I do not think this recent closure is evidence of such a surge, because Dems were in a majority of samples in July, then became a minority, and now are majority again. The registration drives were ongoing all along. The Dem majority should have grown consistently if they were consistently out-registering Republicans.
We'll know more in a week or so.
One problem with a surge in registration, is that the surgers tend not to vote, maybe because a fair number are fraudulent, and generated by those paid by per registrant. In the end, it takes some guts to fraudulently vote, and I don't think many do it. What's in it for them?