Pat Caddell was going nuts about a poll a week or so ago that had a sample with 44% democrats to prove, iirc, that Hillary is doing just fine.
There was a thread here a few days ago that claimed the majority of Americans approve of socialism. When the internals were revealed further in the piece, the dims had a 55% favorable view while republicans were at 37%. In order for the results claimed, the dims would have to be oversampled to achieve a majority (50% +1) overall.
Who determines the polling samples? The people who want to get the results they are seeking. That could be either the pollsters or the people commissioning the polls.