CNN itself states they oversampled Republicans to get those numbers.
Polling Center: CNN Poll: Who won the VP debate? - Elections & Politics from CNN.com
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SPECIAL NOTE OF CAUTION #2: The sample of debate-watchers in this poll were 31% Democratic and 33% Republican. That indicates that the sample of debate watchers is about eight points more Republican than an average CNN poll of all Americans, so the respondents were more Republican than the general public.
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So, shift 8 points ... yet another poll showing a solid Biden win.
Looks like BluePhantom pooched it yet another time, continuing a near-perfect record of getting everything wrong. Due to his prediction, we can be absolutely sure polls will keep swinging towards the Democrats.