Originally Posted by FD
Pollsters don't choose the number of Democrats or Republicans to include in their sample. That is a fluid, outcome variable just like which candidate people support.
Some do, some don't.
Pollsters can, and often do (especially when trying to make projections based on their believed electorate), filter through results and/or drop randomly selected samples if they don't fit the projection model.
The guys that are trying to really predict will absolutely tailor their sample to fit a projected voter pattern, otherwise it's just random chance.