This year out of 100 political poll calls, only about 8 people answer & complete the surveys. Bias due to non response is calculated by:

BIAS = RESPONSE RATE *(ACTUAL RESPONSE- REPORTED RESPONSE)

Twice as many voters who support Trump say they keep their support to themselves than Biden supporters. Assuming Republicans who inevitably respond are still nonetheless Trump supporters, this affects Independents the most. Right now about a third of all voters identify as Independent so I did a back of the envelope calculation:

.92(66-43)/3. This equals 7.1%.

This means Trump could potentially have 6-8 points more support in the national polls than projected. In other elections the number of Americans shy about their support has never been 2:1 so normally this process is washed out. Republicans usually outperform expectations by 2% because conservatives are less reluctant to even admit their politics